Climbing, swimming, weightifting among other things. Even just taking a short walk can do the job for me. I've noticed that just about anything that gets me away from my lonely apartement works, although at times I also enjoy my solitude. When I'm alone I tend to spend my time watching movies, reading books or playing games.

I've also been thinking about buying some music production software and try to create some music of my own. It would be cool to be able to spend the time I'm home alone on something productive, plus it would be interesting to see if I would be able to re-produce some of the tones that are flying around in my head. The downside is that software like this is very expensive, and probably very complex to use. Oh well, it's just a thought.