Guitar and music in general. I've only been playing 3 years with no lessons, but between playing, writing, and transcribing, I've learned I lot. Like, what, knowing, to utilize the harmonic minor's b7 over the V in an Andalusian cadence and then modulating to the relative Lydian or something, I don't know, something complicated sounding like that. I'm still not very good at sight reading, but if I could get a jazz or classical teacher, it would be a godsend for me. Those two worlds I can't really crack all on my own.
I'm also teaching myself Japanese. Don't call weeaboo, I started long before I ever got into anything anime (only a few months ago). I just heard it was hard so I wanted to see how hard it was, and I had millions of hours of free time, and I always wanted to learn a second language after the canadians schooling system raped me over with French. And it's found it pretty interesting. And just think of the practical use! I mean, French? Italian? Spanish? Who would ever need to use those in North America, m i rite?
I also realized I'm big on science fiction, and that's only because I never really realized how much of the things I liked were in fact regarded as science fiction, and are usually shafted by a lot of people for some reason. No cheesy shit or Star Trek shit, though, but that's a given.