That's a lesson I took a long time to learn - that I needed to do things besides music - and I ruined a couple of relationships in the process. That being said, the fact that my wife is a professional musician certainly helps us find a balance.
I guess my main thing besides music is books - religion (especially Judaism), philosophy, psychology, languages and death. Anything by Nietzsche. And classic literature.
My home studio doubles up as a study room, once I'm done recording late at night. Put on some classical music, pour a glass of wine, get some olives, a book and I'm good for hours...
I've always been a bookworm, usually reading 4 or 5 books at once. Right now, I've just started Dickens' Great Expectations, I'm in the middle of my Jewish study bible and a couple of books about Jewish high holidays, I'm reading one called Dictionnaire de la Mort (dictionary of death) and another one titled Funerals to Die For.
Oh, and Marus Aurelius' Meditations.
We also seem to be collecting cats. But that's a different story...