As long as it's interesting or rewarding, anything goes.
My preference would be for something that can be experimented with, or applied in practice to solve a problem. For example, won't mind struggling with some abstract math as long as that will help with implementing some cool DSP effects. OTOH, studying chess, or string theory, or collecting coins won't be my kind of fun.
Not interested in blogging, or in showing what I'm working, either.
Whenever I do that, it usually takes longer than the activity itself.
I guess what I'm seeking the most is the beginner's hype of discovering something new.