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"
I'm a leftie too, also as I understand my grip is wrong, I'm a sidewriter. Writing with a fountain pen is still easier for me, you just have to use smaller nibs to avoid smudging. But yeah, calligraphy and pretty handwriting is not my strongest suite. You don't have to write nice though to use FPs. Just write :3