Given that you say you are limited pretty much to staying home, then it really boils down to an area you would like to study.

I'll leave aside musical endeavours, because as much as the idea of taking the time to record a few numbers on a home studio appeals personally, it's dependent on already having reached some level of musical aptitude that you're happy to make recordings with (doesn't have to be anything for general release, can be just for wife and kids birthdays, family and friends, people appreciate the personal touch with music).

The one area that I am professionally interested in, but don't feel I have the time in my day job as a production DBA, is to look more into statistical computing, particularly in relation to data mining. It's been decades since I did maths at high school and university, so I could do with going back and revising my statistical maths and then going through Bayesian analysis, Markov chains, all that good stuff.