I'm lucky enough to work within 5 minutes of a gym. So, instead of going out to lunch or just killing time, I head over and get a good 30-45 workout it 4 or 5 days a week. I eat my lunch when I get back to my desk.
Working out at lunch is a great way to break up the day. I like getting back to the office refreshed and ready to tackle the rest of the day.