Greetings everyone!
Since there isn't a complete achievement guide yet to 80 days, I thought there should be one. The wikia page[80days.wikia.com] of 80 days provides almost all the information you need to achieve 100%, however that way takes a lot of searching and browsing. You can get the achievements in any order but connecting them helps to avoid repetitiveness in case of a few failures.
The guide also contains some spoilers, I tried not to expose too much, though.
(Unlike Fogg, english is not my native language as you may observe reading the guide.)

Finish the game at least once, the time of arrival doesn't matter. I recommend to do a slow travel to explore a bigger segment of the map (and you can do the 30 Cities and No Banks as well this way, or even a few more too).
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There are more ways to make it under 40 days, I used one of the northern lines:
London - Paris - Vienna - Warsaw - Minsk - Moscov - Karimskaya - Pyongyang - Manila - Acapulco - New Orelans - New York - London (without enough money (4000£) you can't go straight to London from New York as well which saves way too many days to let it go).
Going to Urga from Irkutsk and using the Garuda to Yokohama also works so you can continue to Honolulu if you explored the experimental hoover from the news. It is essential on this route to speed up the Trans-Siberian Express, to do that, you need to talk about the line with the Russian gentleman on the Gallant Rider to Warsaw. After you got the information, talk with the engine driver on the Trans-Siberian Express and try do a wager about the train's speed. He won't accept it the first time so you need to follow the red haired lady (Roza) on the train and tell the driver someone's hitting on her. Of course, he'll accuse you but tell him you're innocent and this way he's going to speed up the train (you can't depart at Urga after this).
Following the London - Paris - Istanbul - Tehran - Kabul - Agra - Calcutta - Singapore - Manila - Acapulco... route can also work.
Overall, airships are always the fastest but the most expensive lines. If you're aiming for 40 days you'll probably need to try it a few times because every new time (restart does not count) the game generates new market items. However, there are a few items you can find almost every time with very little difference (the wine in Europe for 2-3k, the tin sheet/something else for 6-7k in Brisbane, etc.)
Do not use banks during your travel. Without any time restrictions, it is quite easy, just buy and sell a few things at the proper cities.
You can't use hotels during your travel. Sleeping outside hits Fogg quite hard (the amount depends on the climate of the area) so you need to travel long distances with the proper equipment. Probably the easiest method is using the Orient Express and the Trans-Siberian Express and going to Honolulu where you can get a full regain for free (walk on the shore with Fogg, eating fruits inside heals less). From there, use one of the North-American routes and an airship to London/Reykjavik(/Nanortalik). Side note: always use your valeting skills (the comb) during the trip to heal Fogg, it makes the achievement much easier.
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No time restrictions, so you only need to be creative. The easiest method of course is to stop in every city where it’s possible, this way you can explore and memorise routes, events, common trade items.
Depends on the length of your routes, this achievement takes about 8-10 playthroughs.
Probably there are more ways to do this, the fastest is going to the North Pole without proper equipment and healing Fogg. The Icewalker always crashes so you only need to keep Fogg’s health low and do not ask the others for help (food, buffalo hide, healing potion).

Befriend Goland on the Trans-Siberian Express (on the way from Omsk to Irkutsk). If you success, she will give you a Mongolian Sulde which you can try to use in Urga to meet her in the temple complex.
Make your way towards South-America, to Lima. There you can meet Mr. Quispe whose plan is to make some nice pictures of the Macchu Picchu’s aerial view from his gyrocopter. Go with him to Tabatinga, stay there for a few nights, that way an event will unlock his gyrocopter to Bogota too. Take his ride again, at the end of route he’ll offer to take a picture of Fogg and you to the newspaper which will unlock the event at the end. Finish the game and Goland will be waiting for you in the crowd.

In Calgary (confirmed) or Regina (did not tried) an event will trigger where you can obtain a mysterious device in an inevitable fight. Just pocket and keep the thing, it will unlock a new route from the town with airship. Ride the ship, they’ll abduct you soon and will be held hostage on the Kahwoka for a trial where you’ll be found innocent.
See above. Obtain some whiskey on your travelling so you’ll be able to lie after your abduction that you’re whiskey traders (which is forbidden there) so the trial will found you guilty. After that, you can steal a ship and escape from the floating island with landing on the river just before the Niagara Falls.
The Black Rose storyline needs some luck and well-timed triggering. On the Amphitrite Express, help the man with his lost silver cigarette case (if the event is available). Ride the Blue-Line ferry from Nice to Tunis and stick to the girl named Estelle (SPOILER:yes, she’s the world famous thief) until Luxor or Beirut. Pick one of the Indian routes, if you’re missing something or gifted with something (money/rose) you’re doing good. Somewhere in India the Black Rose will visit you and tell you about her plan to steal from the Artificers Guild. Agree with her and promise to help her. The next step is Buenos Aires. According to the wikia, it is possible to go from Asuncion or Santiago but those way did not activated the event for me in Buenos Aires, only from Rio de Janeiro with the fake news about the capture of the Black Rose (you have talk with the captain, he’ll be the one informing you about the details). If you succeed the Black Rose will join you in the lift to the upper city (when the sound asks for your rank, do not lie) and thus you’ll be able to ride the El Dorado together. On the ship, explore everything and kill the time until you meet an injured robot. Help it and ask it what has happened to its leg. Following the robot’s guide you’ll find a hatch. Open it with the Black Rose and enjoy the ending of the (game’s probably best) mini storyline.
See above. If you led the Black Rose to success, there will be a chance to join her in her adventures. Accept it, along with her unique ‘lifestyle’ to gain the achievement.
Similar to the Goland-line with only one small difference. Head to Smeerenburg and encounter Vitti Jokinen. From now, always pick the options involved with Vitti because you need all of the events to build up a romantic relationship. Be sure to keep Fogg alive near the North Pole. On the Pole, stay with Vitti when you can, he will disappear soon. Your next (and final) destination is Nanortalik where you will meet Vitti again. Comfort him, prove your love, and you’ll be able to let Fogg finish the rest of traveling alone.
Head to Madras and meet with the Sisters of Didacus (you need to venture out and trigger the event with not escaping). Sister Panimalar will reveal their plan to find an automaton with a soul. In case you had the Mysterious black cube from Bucharest or a Venetian glass from somewhere else, do not give it to Panimalar, it would cut you off from the achievement. Go to Timbuktu (it’s up to you how you get there) and ride the Ghazal. On the Ghazal use Passepartout natural curiosity to meet Aissatou, Madame Diallo’s artificial daughter. When she’s cold, look for her mother, you’ll be given Aissatou’s old shard as a payment for your silence. In Timbuktu, go to the station and send a telegraph to Madras but mention the whole truth not just the shard (the full betrayal is needed for the achievement). The Sisters will send you ten thousand pounds for your help, use it to fasten up the remaining part of the trip (so Diallo won’t be able to hunt you down for your treachery).
Travel to Kristiania, explore the underground town then meet with Otto Lidenbrock. Otto is looking for a way to the core of the Earth so be a true gentleman and suggest him Reykjavik. Travel around the globe and fly to Reykjavik at the end. In Reykjavik you’ll meet Lidenbrock again and he’ll offer a new way from the town. Follow him and you’ll find yourself in England with the achievement as well.
Your first stop is Minsk where you can meet a man in a tramstop who reveals his old love, Marina Poltavka’s true identity. Head to Novorossiysk, board the Blue Flower and get in contact with Marina/Leena. You can assure her that her old friend’s love toward her was quite clear even though you merely talked with him for a few minutes. Marina will redirect the ship to Mount Elbrus, granting a faster travel method and an achievement.
Aim for the southern corner of Africa, Bhayi to be exact. Talk with the locals, listen to them so you can board the Tokoloho, a ship which intends to hunt down a sea monster. From there you’ll get the achievement plus a nice plot twist.
Board the Orient Express, travel to Vienna where you can offer a lot of wine to Herr Danzer soldier from the steam orchestra. Guess how he controls the automatons with sound and you’ll be able to steal a Zauberflote from him. At night go to a bar so you can overhear that the Viennese Military Transport is going to Istanbul via Belgrade. Go to Istanbul, disembark at Belgrade where you can find and help the locals by giving them the Zauberflote to control the automatons.
See above. After getting the Zauberflote keep it for yourself. Head to Lisbon where you can meet Herr Danzer once more just to fully ruin his life.

On the Transcontinental Express between Omaha and Burlington you’ll confront with Jesse James and his outlaws, demanding the wealthy passenger’s belongings. A good valet not only cares about his/her master but also protects it so help Mr. Fogg out in the situation and you’ll be awarded with an achievement.
In Rangoon you can meet with Manussiha, a guardian automaton who asks you (with quite cryptic) to help an innocent man and send you to Pangsau Pass. In Pangsau Pass you’ll have a chance to save Surya, a revolutionary from execution. Your success (SPOILER Do I smell fire?) also opens a flight with Garuda to Beijing (or Canton with losing your possessions if you fail to save Surya).
See below achievement 35. Start the investigation and use the equipment to witness the abduction and discover a new airship route to Manama. On the route you can save Helena and get the achievement.
Ride the Piedmont-Airline from New Orleans to Washington where you’ll have a chance to fight with professional boxer Charlie Sullivan. Fighting itself is not enough though, you also need to win the fight. During the fight, you have 3 options:
a) block
b) jab
c) punch
Block is effective against punch, jab counters block, block wins against punch. Follow Fogg’s orders and check the narrative carefully for Sullivan’s next move. If you win, you get the achievement with 2000£ which can be crucial for the 40 days achievement too.
To get this achievement, first you need to read the newspapers often to discover the experimental hoover from Yokohama to Honolulu. Before boarding the Noelani you’ll be given a monkey wrench/gun/katana by a stranger which you’ll find at a murder scene. To solve the case you need to SPOILER discover the steam-valve at the scene, talk with Mr. Jaffer about the estimated time of death and interrogate General Peters for the information about Ms. Carlotta’s business interest and relationship with Madame Shu. After getting all the information, and giving the proper answers (SPOILER steam-valve, perfect timing, business), confront the killer (SPOILER Ms. Carlotta) as a finish of the investigation.
You need to finish the game a few times first before the prerequisites of the achievement can be unlocked. When you get the chance to travel to Zurich (usually there’s a news about a rocket scientist), go for it. Wait for a few days to meet Mr. Ardan and get a fast travel to Tunis where plot unfolds. Ardan plans to make a rocket big enough to go for the Moon in Miami you will need to reach in time, so make sure to minimize you travel time. From there you’ll only need to travel to Nassau and enjoy the rest of the story.
Travel to Istanbul with the Orient Express. Nn your way help Henri de Blowitz with his bags and sit with him in the dining car to discuss the political tensions of the Austro-Hungarian Empire. Ship down to Alexandria and travel to Cairo from there to find Blowitz again in the souk where he asks you to keep an eye out for bigger automatons. In the nearly Aswan you can find one guarding the city after the curfew so travel there. You can contact Blowitz in Wadi Halfa at the telegraph station to get the achievement.
I recommend doing this with the one above because it starts at the telegraph station in Wadi Halfa where the other storyline ends. For a few bucks let the kids dictate their message (‘Ulundi Passepartout’) to the writer which triggers new events. Travel to Ulundi where the local leader organizes a meeting with the Artificer’s Queen in Antananarivo. Ranavalona II provides the Maminirina, her Majesty’s special airship for a fast ride to Rangoon. With a small investigation you can unfold the Maminirina’s secret and get the achievement with a gentle kiss.
All you have to do is to visit the World’s Fair while you’re in Paris.
From Brisbane, you can ride the a submersible named Leyden. At the end of the road Captain Rosencrans offers Passpeartout to dive in the Great Barrier Reef. Accept it to get the achievement, also pick up a worthy Coral Sphere.
Travel to Alexandria and explore the library where you meet Helena Kapodistrias who shows Passepartout an invention which can show the past for the user. Go after Helena to Cairo where you’ll meet her again and stay together in a hotel, the next morning she's going to be vanished. You can start investigating and find her equipment but do not use it, just simply take it otherwise you won’t be able to use it later. After getting the invention head to the Pitcairn Islands and explore the cave where you’ll be able to use the device. The device will show the path which leads into Macchu Picchu.

Thanks for the reading, I hope the guide helped you in your adventures. Feel free to rate, comment or ask.
As it is my first guide, any feedback would help me to improve.