You're a programmer? Go for a walk at lunch. It's good on two fronts - it gets you a break from the office (and co-workers), and it gets you exercise. (Depending on where you live, December may not be the ideal time to start doing this...)

One nice thing about a walk is that you can go for 10 minutes, or you can go for an hour. You can start small, and as you get in a bit better shape, you can go longer.