Obsidian. Very simple, all functionality is free, offline, all notes are accessible on your PC, you don't even need Obsidian to open them because they are in Markdown. Has tons of community plugins for every need, even a Notion-like tables plugin. There is an android app which looks fine but I don't use it much so I can't say anything about it. But since all your notes are offline you have to think of ways to sync them. There is a paid Obsidian sync which you dont have to use or you can sync with any other method (manually, github, syncthing, etc...)
Edit: Oops, just realized the post is in FOSS community and Obsidian doesn't seem to be open source. Sorry! I'm not going to delete the comment though, maybe it'll help someone
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"