The first thing you can do is park a little further away from your office. Heck, you can do this even when shopping. Don't always look for the closest parking spot. A fringe benefit to this is that you don't have to look hard for a spot or worry about the driver of the car next to you denting your car.
Also, take the stairs rather than an elevator. Stair climbing is an excellent form of exercise. Taking stairs throughout the course of the day equals extra calories expended. According to healthstatus.com, in the minute it may take you to stand in the elevator you can burn up to eight times more calories walking up the stairs.
Occasionally, when you aren't in a hurry, rather than calling someone in your office building on the phone to ask a question, go to their office and ask them personally. Not only does this provide an opportunity to exercise, but a chance to visit. If this is inconvenient, simply standing while you're on the phone will burn more calories than sitting.
How about turning a small meeting into an exercise session by "walking and talking?" This probably won't work with large groups; however, when only a few of you have things to discuss it may be a perfect opportunity to move around. Studies show improved brain functioning when you are physically active. Walking and talking may stir up your creativity!