Really like most of the hobbies from comments, you guys are interesting!
Mine are (I'm not sure if they are niche but):
Fountain pens. Writing with them feels very nice and smooth and it's a little piece of "retro" even though I don't have time for any other kind of "retro"
Lucid dreaming. It's an exercise of control over your dreams and a chance of doing something you like when you sleep. Or experience something new. There are forums with quests like "go to an art gallery in a dream and explore what your brain can give you as a painting" or "build yourself a dream homebase with all your favorite pieces of dreams from childhood" or "jump into a chalk painting and describe your experience"
Yeah writing down my dreams is hard for me too especially when you just woke up in the middle of the night. But i found that it helps if you don't write it down in every last detail. I only jot down a few keywords when I wake up and that usually works, then I can flesh it out it later in the same day when I have time. Takes less effort. If you don't remember - guess, that's a good exercise too! Don't do it tomorrow though, never works :'D Also it can help if you're forced out of the bed somehow for a couple of mins (drank too much water and have to go to the bathroom, for example), then just take your journal/phone with you. It also helps with WBTB (wake back to bed) by the way.
I don't know is links are allowed but here's the forum with fun dream tasks I mentioned: https://www.dreamviews.com/tasks-month-year/
I don't read a lot of forums but this one motivates me because users create this witty unexpected tasks I could never think of myself. And you get to feel accomplished when you finish them XD