Ruins:
- The first one is shown in the preview image for the mod
- Be very tiring
- Avoid taking damage

Snowdin:
- Get a red flag on the ball game (If you're struggling with this, lose a few times then talk to Sans and he will tell you the secret) (This only works if you've not killed anyone.)
- Enter a dark room
- Get really lucky

Waterfall:
- Find a treat under a bench
- Locate an artifact
- Read a diary

Hotland:
- Remember
- Walk on magical glass
- Solve the tile puzzle before time runs out

New home:
- Get to the judgement hall in under 90 minutes.

Easter eggs:
- Help nature (This one is somewhat obscure. It involves a water cooler. Good luck.)
- Do a quest