What do you do to keep fit ?

What do you do to keep fit during the limited time between classes and study ?

I hit the gym, but my classes happen to be at the gym so it doesn't realy count, but I also do bodyweight exercises at home.

I pretty much power walk to class. My boyfriend makes fun of me all the time and says it looks like I'm running. But it's cold out and my classes are really far away so I don't have much choice - I might freeze to death! But I mostly just walk a lot and try to eat healthy. I'm not too much of an exercise buff because technically I'm underweight and I'm terrified of losing more. So anything I do is strength in the hope of gaining a little muscle mass.