What do you like to learn about in your spare time?

I don't even think about it this way. The time I spend learning is not "spare" time, that's core to everything I am and everything I do. The rare bit of "spare" time I allow myself I usually either go fishing, read fiction, or watch movies/television.

All of that said, I have a huge (too huge) list of things I want to learn, or am "working on" learning to one extent or another. There's a huge amount of techie stuff, mostly related to AI, a lot of math stuff, physics/cosmology, etc., as well as "non techie" topics related to business, economics, history, philosophy, and FSM-knows what else.

I could spend 24 hours a day, 7 days a week studying and learning and still couldn't put a dent in the list before I die.

Usually history or whatever subject interests me at the moment. Once at the airport I had an obnoxious layover and it was way too early to hit the bar so I ended up reading the data sheet for some computer that controls some terminal door.

I want to learn musical instruments. But not sure how to start as an Adult.

Reverse engineering and taking 5-6 courses to reach a mathematical understanding of General Relativity.

Actually I have so much on my plate.