My favorite part about flying is that I use it as an excuse to focus on small personal projects. Last night, on my red-eye flight back from west coast demo day 2008 (oh yeah, in case you didn't know, I am in YCombinator Summer '08, co-founder of MeetCast) I made a custom RSS reader for my blog. This isn't any ordinary RSS reader, however, it is in the form of a physics sandbox. You can see it in action here.
It's still pretty rough, but is still fun to play around with. I used the as3 version of Box2D for the physics, which I am really impressed with. Eventually I will add more parameters besides gravity, as well as more feeds (e.g Twitter).