Hi! I decided to write this simple guide to help fellow completionists unlock all the achievements in the game. I was surprised to find out no such guide existed yet, as this is such a cute and charming game. Hopefully this will help someone out, even though it's far from perfect (and could probably use some improvements).

Rather self-explanatory, you'll get these achievements just by playing the game unless you're trying to avoid them for some reason. Refer to the "Inn Ratings" section of this guide if you're having trouble getting access to the Cave or Ruins, as they are requirements to achieve additional stars for your inn.


My First Guest
Check in your first guest.


My First Ad
Design an advertisement.


My First Reservation
Reserve a room for a guest.


No Vacancy
All five rooms at the inn are occupied.


Country's Best Inn
Achieve an average guest rating of 140 or higher with at least four rooms occupied.


Remarkable Ruins
The ruins are open.


Splendid Spelunking
The coral cave is open.

All the requirements to get additional stars for your inn are detailed here. None of them are particularly hard, but I'm gonna list the solution for each of them in case you get stuck.
The "Earn x gold / guest rating" ones are pretty self-explanatory and should come easily as long as you're doing your best to make every guest happy.


One Star
Receive a one-star rating.


Requirements:

• Get rid of rats.
  - Lance will do this for you during Day 2 as long as you can provide a room for him to stay in.
  
• Get rid of spiders.
  - Robin and Sparrow will show up during Day 4, book a room for them and they will exterminate the spiders for you!
  
• Average guest rating above 35.

Two Stars
Receive a two-star rating.


Requirements:

• Hire a maid.
  
• HIre a chef.
  - Kathryn and Emily will show up in the manor during day 6, simply book them in and achieve a rating greater than 0 to convince them to work for you!
  
• Have 4 guest rooms.
  - After Rudolph visits the manor for the first time, Frank's house will open. Go talk to him and convince him to repair the manor to achieve this!
  
• Earn at least 180 gold in room revenue.

Three Stars
Receive a three-star rating.


Requirements:

• Hire a receptionist.
  - During day 12, Beth and Meg will move into the farm north of the manor. Talk to Beth inside the house and she'll offer to work for you in exchange for a telescope. To get it, help Nobby upgrade his store and he'll sell you one for 150 gold.
  
• Unlock the wine cellar.
  - To unlock the wine cellar you'll need two things, a hammer and some grease. The hammer is found in Frank's house (hanging on a wall), while to obtain the grease you need to help Nobby upgrade the store by lending him 1000 gold.
  
• Earn at least 120 gold in tips.

Four Stars
Receive a four-star rating.


Requirements:

• Hire a gardener.
  - Tim will show up in the inn as a guest, simply give him a room and he'll work for you as a gardener!
  
• Hire a bellhop.
  - To hire Remus you first need to unlock the ruins. In order to do so, you need to help Meg grow her crops. At some point she'll ask you to help her with a wolf problem. Simply hire Lance and he'll get rid of it, finally unlocking the ruins. Explore them and you'll find Remus, talk to him and he'll work for you as a bellhop!
  
• Furnish the lounge.
  - After you unlock the wine cellar, go speak to Frank about the manor's building plans. Go back and you'll find an hidden room full of items to furnish the longue with, but you'll need to hire both the gardener and the bellhop in order to move them.
  
• Earn at least 500 gold in profit.

Five Stars
Receive a five-star rating.


Requirements:

• Hire a boatman.
  Book Charon for a room in your inn and he'll offer to work for you as a boatman. Remember to disable pets in your inn as he doesn't like them and won't stay unless you do.
  
• Unlock the suite.
  In order to unlock the suite, you need a doorknob from Darius, for which you need to unlock and explore the cave near the farm. To open the entrance you need to book Robin and Sparrow again. For them to show up, you need to make spiders come back to your manor. Simply do not hire the maid for a couple of days and they'll infest your inn again!
  
• Earn at least 1,000 gold in profit and achieve average guest rating above 100 with all 5 guest rooms occupied.

These achievements are missable and require specific choices or actions throughout the game to get. They are listed in the order you're most likely to unlock them. Some of these exclude each other, but you can get all of them in one playthrough as long as you save and load at the right time.


Pays to be Nice
Get the maximum possible discount from the carpenter.


- Right after the tutorial, George will come into the manor and ask for a room to stay in for the night.
Choosing the last option "You poor thing. Of course you can stay the night." will grant you the biggest discount along this achievement the morning after!

Free Lunch
Get the manor repaired for free.


- When you talk to Frank about repairing the manor, ask him to go lower on the price until he offers to fix it for free. This will later help with the Outsmarted achievement as well!

Altruist
Give the shopkeeper money to purchase supplies, with no strings attached.


- When Nobby asks you for 1,000 gold to upgrade his shop, choose the option "I'll happily give you the money - you don't have to pay me back!" to get this achievement. This will also shorten the time it takes for him to upgrade his shop by 1 day!

Pet Lover
You told Wodehouse you loved pets.


Not a Pet Lover
Because you said you didn't like pets, you received a very valuable item during the surprise party.

- During Day 12, you'll have to distract Wodehouse with a fishing rod and get into his room.
After he comes back, he'll ask you if you like pets. Saying yes will grant you the "Pet Lover" achievement right away, while saying no will unlock "Not a Pet Lover" achievement only after you repay the loan and go through the surprise party scene at the end of the game.

Ghost Buster
Chase off the ghost.


- During Day 14 a ghost will visit the inn at night. Listen to what the guests will tell you during checkout phase to find out which room the ghost visited, keep it vacant and hide in it the next day. You'll get a clue that points at Rudolph being the "ghost". Confronting him and you'll get this achievement!
WARNING!Choosing to ban Rudolph from the manor after exposing him will lock you out of the "Outsmarted" achievement, be careful!

Merry Merchant
Receive a gift from Johan.


- After booking Johan for a few times and making him happy, he'll gift you one of three items. Choose the one you like the most and you'll get this achievement!

Matchmaker
Lance and Gwen met and fell in love because of you!


- Whenever Lance and Gwen show up at the same time, book a room for each of them. Eventually, one of them will ask you for advice on how to propose the other, help them out and unlock this achievement!

Vibrant Village
All buildings in the village and the fountain have been upgraded, occupied, or built.


- Here's a list of all the things you need to do to get this achievement:

• Upgrade Nebby's store
  
• Upgrade Iolanda's cafe
  
• Upgrade Darius' blacksmith shop by giving him the coal found inside the cave.
  
• Repair the fountain by booking Kyle and his family and talking to him during leisure time.
  
• Build the Train Station by achieving 3 stars before day 15
  
• Help Meg grow her crops
  
• Have Yuna move into the village (story related)

Buried Treasure
Obtain the painting of the couple.


Mystery Solved
Solve the mystery of why Aunt Agatha had to borrow money, and discover Rudolph's deceit.

- In order to solve the mystery, you need to find all the paintings that Aunt Agatha hid inside the manor.
There are 4 in total:

• Weaver at Work, found under the rug in the manor's 1st floor
  
• House with Rose Bushes, found inside orange room's fireplace and recovered by booking Neko in it for a night.
  
• River Flowing Through a Field, found inside the secret room in the wine cellar
  
• Painting of a Tree, found inside the Suite

Once you collect them all, you need to analyze them with the magnifing class found inside the wine cellar. They will reveal various places around the village you need to investigate, these are:

• Yuna's loom
  
• The middle rose bush on the left just outside the manor, you will need a spade for this one which is found inside the cave
  
• A rock in the middle of the river outside the manor. To reach this one, you'll need to hire Charon and talk to him during leisure time

Doing so will yield a blueprint for a key, bring it to Darius after upgrading his shop and he will make it for you after a single day.
Once you obtain it, you have to head on the hill on the left side of the farm by going through the cave, once there you'll get a prompt to dig up a chest. Open it with the key that Darius made for you and you'll unlock both of these achievements!
WARNING! Waiting too long to solve the mystery will lead to Rudolph finding it before you, locking you out of these achievements. Read the "Outsmarted" achievement description to learn more about it.

Goodbye, Rudolph!
Expose Rudolph's villainy and run him out of the village.


Beautiful Roses
Get Rudolph to give you the painting of roses hanging on his wall.


- After solving the mystery of the manor, you will have to confront Rudolph about his villanous deeds. You can choose to either expose him and force him to leave the village (this will net you the "Goodbye, Rudolph!" achievement), or ask for the painting hanging on his wall in exchange for your silence for the "Beautiful Roses" achievement.

A Good Deal?
Sell paintings to Rudolph.


- Near the end of the game, Rudolph will ask you to sell him the various paintings you'll find in the manor. Simply accept and you'll unlock this achievement.

Adventurous Eater
Willing to try new foods.


- For this achievement, you need to hire your chef as often as possible and talk to her during leisure time. Sometimes she will ask you to try out a new kind of food. Do this enough times and you'll get the achievement!

Lost and Found
Return 10 lost-and-found items to guests.


- Hire your maid as often as possible and talk to her during leisure time. She will sometimes give you items that your guests have lost. You might get the option to return them to the guests who have lost them during check-ins, do it enough times and you'll get this achievement!

Pranked the Prankster
Play a prank on the prankster.


- Shortly after you buy a phone, you will be able to reserve a room for Joe, Robin, April and Lou. Doing so will end up in them not showing up. Reserve a room for them a second time and you'll figure out it's a prank.
Investigate anyone in the village with a phone and you'll find out it's Frank.
Confront him and choose the option to take revenge by playing a prank on him yourself.

Repeat Customer
A guest stayed eight times before Day 30.


- Pretty self-explanatory, Lance and Irene are the easiest guests for this one as they show up very often and don't require much to book, but I'm sure you can achieve this with a few others as well.

The Royal Family
Play host to the royal family.


Royal Warrant
Fulfill all of the royal family's needs and receive the Royal Warrant.


- Near the end of the game, the royal family will visit your inn. During the check in, they will ask for advice for a specific activity. Point them in the right direction (and have said building / activity upgraded) to get both of these achievements the morning after.

Full Ensemble
All four opera singers stayed at the inn on the same day.


- After you book all the opera singers at least once, they will show up at the same time during day 27. They are Ulrica, Igor, Siegfried and Susanna. Give them all a room and you'll unlock this.

National Treasures
Give the gold sword and gold plate found in the ruins to the national museum.


Private Collection
Keep the gold sword and gold plate found in the ruins for display in the manor.


- After unlocking the ruins, you can book Idaho in order for him to explore the ruins. Meet him there and he'll talk to you abou the missing pillars. They're found inside Frank's house, talk to him and if you pay him he will put the back in their place.
Book Idaho again and this time he will tell you about a treasure found in the ruins. Find it and you will be given the option to either sell it to a museum or keep it for your private collection.
WARNING!Selling the sword to the museum will lock you out of the "Innkeeping Expert" achievement.

Art Connoisseur
Talk to the art expert and keep the six paintings owned by Aunt Agatha.


- After you reach 5 stars and got all the paintings, advertise your hotel in the best way possible. This will get the attention of Pablo, the art expert. He will offer to buy your paintings. Do not sell him any and you will get this achievement.

Golden Goose
Make a profit of 1,500 gold or more in a single day.


- This refers to the profit from booking and tips at the end of a day, selling items do not count toward this achievement.
Simply activate as many aminities as you can and book every room with the wealthiest guests. Play around with the price level but make sure that every room is occupied.

Throughout the game you will be able to find 15 different books scattered around the game's world. They function as collectibles and you can choose to either sell them or keep them to give your inn a boost once you got the complete collection.
Here's a list of all of them and where to find them in order of accessibility.

Book Collector
Find all 15 Dickens books.



1. Dombey and Son (7th Book) - Found inside Iolanda's cafe, on the table by the entrance.



2. Hard Times (10th Book) - Found on a crate in Darius' shop.



3. Martin Chuzzlewit (6th Book) - Found inside the cabinet in the manor's lounge.



4. The Old Curiosity Shop (4th Book) - Bought from Nobby at his shop.



5. Nicholas Nickleby (3rd Book) - Found inside the manor's library.



6. Our Mutual Friend (14th Book) - Found on a barrel inside Frank's house.



7. Great Expectations (13th Book) - MISSABLE This book is found inside Rudolph's house and is only obtainable when he asks you to stay in your inn, break into his house through the window and investigate the top left bookshelf.



8. Little Dorrit (11th Book) - Found on a shelf inside Meg and Beth's house.



9. Barnaby Rudge (5th Book) - Found on a barrel inside the manor's wine cellar.



10. Bleak House (9th Book) - Found inside Woden's room, accessible after distracting him with the fishing rod.



11. David Copperfield (8th Book) - Found on a cabinet inside Yuna's shop.



12. The Pickwick Papers (1st Book) - Found inside the wine cellar's secret room piano.



13. Oliver Twist (2nd Book) - After hiring Remus, talk to him during set up / leisure time and he will give you this book.



14. The Mystery of Edwin Drood (15th Book) - Found on a crate inside the cave.



15. A Tale of Two Cities (12th Book) - After solving the manor's mystery and digging up Agatha's treasure, you will find the last book inside among other things.

Achievements that have to do with following a certain playstyle or reaching a specific ending.

Owner of the Manor
Pay off loan.


A Challenging Month
Pay off loan in classic mode.


Good Value for Money
Pay off loan without raising prices beyond the 'Cheap' level.


Innkeeping Expert
Pay off loan without selling anything found during the course of the game.


- I'd suggest going for these four achievements at the same time, but you can also achieve these in two different playthroughs or more to make it easier.
Paying off the loan without selling anything / raising prices beyond cheap isn't that hard and doesn't require NG+, just make sure to always book as many people as possible and fulfill all their likes. Invest in aminities, upgrade stores and earn stars as soon as possible.
I'd suggest saving during set-up time and proceed to check-in as to know exactly how to set up your rooms until you get a phone. Good luck!

Co-owner
Pay off part of the loan and reach a profit-sharing agreement with the bank.


- In order to achieve this, you need to reach day 30 with an amount of gold between 12,000 and 19,999, no more, no less. You will be asked to become a co-owner of the manor with the bank, which will unlock this achievement if accepted.
An easy way to achieve this is to get the desired amount of gold, disable all aminities and advertisements and going through every day without booking anyone until you reach day 30.

Outsmarted
Someone outsmarted you to find the buried treasure first.


- To get this achievement you need to let Rudolph find Agatha's treasure before you.
The easiest way to achieve this is to make Frank fix the manor for free at the start of the game. This will prompt Rudolph to show up at the manor during day 25, asking you to let him investigate the supposedly toxic materials that Frank used to fix the walls
Accepting will make it so Rudolph will find the treasure before you after a few days. Head to the hills near the farm, interact with the unearthed chest and you will get this achievement.
WARNING! Banning Rudolph from the manor after exposing him as the prankster will lock you out of this achievement. Be careful!

These achievements are only unlockable once you reach NG+. In order to do so, you need to repay the loan during a normal game. You won't retain anything from your previous playthrough, but you'll unlock a few things that are going to help you out.

Aspiring Architect
Receive a gift from Irene (new game+).


- Simply talk to Irene every time she shows up during NG+. Eventually, she will give you a gift, unlocking this achievement.

Magnificent Manor
Buy all upgrades for the manor from the general store and the tailor (new game+).


- In NG+, you will unlock two new items: the Teleport Stone (bought from Nobby) and the Pet Pillow (bought from Yuna). Buy them along with everything else they sell and you will get this achievement.

Aquatic Visitor
An aquatic visitor and her pet stayed at the inn (new game+).


- This refers to Sirena and Largo. They will show up at your inn only during NG+ near the end of the game. Be sure to keep an eye out for them!

Everyone is Welcome
Every possible guest stayed at least once (new game+).


- Self explanatory, here's a list of the guests you can keep track of:

• Marie and Pierre
  
• Irene
  
• Emily and Kathryn
  
• Jia and Lin
  
• Ulrica
  
• Gwen
  
• Johan
  
• Shirley and Walter
  
• Rudolph
  
• Idaho
  
• Igor
  
• Kyle, Ashley, Nick and Alyssa
  
• Siegfried
  
• Cleo and Anthony
  
• Neko
  
• Tim
  
• Pat, Anya, Leya and Lukas
  
• George, Axel and Otto
  
• Aqua, Terra, Ventus and Ignis
  
• Croesus and Lydia
  
• Lance
  
• Lance and Lupa
  
• Sirena and Largo (NG+ only)
  
• Robin and Sparrow
  
• Robin, Sparrow and Mr Turnip
  
• Pablo
  
• Susanna
  
• Vicky and Alberto

And there you have it.
If you have any doubts / questions about things I might have possibly left out of the guide, feel free to leave a comment and I will try my best to help!