I sit way too much at my desk. I haven't really been exercising since I stopped playing volleyball in December due to injuries. There are a number of people who have moved to stand-up desks - mainly for health reasons (or just to feel better).
A $25 alternative by Oristand that was developed by Hootsesuite co-founder. It is made of durable cardboard. (It might hold an Apple 13" but I have seen people pound on keyboards, so no idea how long it would last.)
It takes a little getting used to. Learn to work standing up.
A buddy of mine has treadmill stations in the office so people can work for 20 minutes or so on a treadmill. That would be cool for a short while.
A blogger associate wrote about his research on stand-up desks. (He picked Ikea).