I will be ranking the sections on 3 things.
1; Fun ranking.
this is how much fun i had doing this.
2; Complexity.
this is how intricate the project is.
3; easiness.
this is how easy it is to do the project.

These will be the main factors to determine how much fun the average person might have doing these. I would also recommend that you do not eat while reading, as these projects will get rather gross. Now, on to the first project!

This project can be done in 3 steps.
Step 1. Find a waft of Gasopods.
Step 2. Swim through their ranks(get it?).
Step 3. Swim away and watch as they release their intestinal gases upon each other.
Fun ranking; 7/10
Complexity; 3/10
Easiness; 10/10

This project will take you 5 steps to complete.
Step 1; Get some metal salvage.
Step 2; Find some Stalkers.
Step 3; Go down to ground level.
Step 4; Drop down your metal salvage.
Step 5; Watch as the stalkers go ramming into the sand.
Fun ranking; 9/10
Complexity; 5/10
Easiness; 6/10

This project can be done in
Step 1; Empty your inventory.
Step 2; Fabricate a knife.
Step 3; Go to the back of the Aurora.
Step 4; Find one of the Reaper Levis
Step 5; Swim around to it's lower back very quietly and carefully.
Step 6; Smack it with the knife.
Step 7; RUUN!
Step 8; If you didn't die, then very well done!
Fun ranking;10/10
complexity;10/10
easiness; a whoppin' 0/10(not easy whatsoever).
(Photo of a Reaper levi from my days at the Crash Zone Research Facility.)

I hope this has given you ideas for things you could do in your Subnautica worlds when bored. Also, if you died doing the Reaper one on your hardcore world then i'm sorry... Please feel free to post your own ideas for projects in the comments. Have fun!