Writing. My ability to write is highly dependent on the context. I'm much better at essays than emails, and fiction is completely outside my abilities. I would also like to change my style a little to be less flowery and more succinct. More Paul Graham than purple prose.
Marketing. I'm hopeless at it, and I need to be at least OK at it in order for my project (https://nuenki.app) to succeed.
Time management. I'm very good at obsessing over one thing; less so at managing lots of different things that need to be round-robined.
Physics and maths! I'm in my gap year at the moment, but I want to be prepared for my physics degree next year. I'm already really quite rusty.
Git. I know enough to use it, but I've no clue what a rebase is, for example.
I've had a vague interest in 3D printing for a while, but I've recently been getting into DND and it might finally justify getting one. It seems like miniatures are best with resin printers though, while most other things aren't. I also hate painting things, so maybe it isn't worth it.
German! Nuenki has distracted me from it, but I ought to properly get back to it. It's just something I enjoy - I've discovered how much fun language learning is.
And a number of other things, but the list is already quite long and I should probably be enjoying Christmas instead of browsing HN :)