1. Don't Sit during Your Entire Lunch Break
Make a point not to stay seated during the 30 to 60 minutes you're allotted for lunch. Instead, take half of the time to eat and use the other half to squeeze in a mini, but mild, workout. Go outside and take a short walk. You can also walk up and down the stairs in your building. This isn't the time for a hardcore workout where you'll break a sweat (save that for the gym). Walking at a moderate or slow pace is still more beneficial to your mind and body than sitting around for an hour. The change of scenery and the little bit of activity is a good way to break up the day.
2. Don't Save Yourself Trips
If you have to run some files upstairs to the accounting department and a small package to the shipping office, don't do it all at once. It may seem counter-intuitive when you're busy and trying to save time, but if you have time to spare, splitting up your office errands forces you to be active throughout your day. Instead of going up and down the stairs once, you do it twice (or more).
Same goes for any kind of inter-office communication you have to do. Rather than calling your colleague down the hall or shooting him or her an email, get up and walk over to his or her office. Any excuse you can find to break away from your desk and move is a good way to stay active through your work day (just so long as it doesn't impair you or your coworkers' productivity).
3. Tag Along for the Coffee Break
Even if you prefer to save your money by drinking the coffee at work (or aren't a fan of coffee in general), taking a stroll with some of your co-workers to the local coffee shop is a quick and easy way to incorporate some more physical activity into your day. Opting to take the stairs instead of the elevator will give you the maximum benefit.
4. Say "No" to the Elevator
Speaking of elevators, make it a habit to start taking the stairs instead. Begin your work day by walking up the stairs to your office and then ending the day by walking down the stairs to the exit. Start to think of the elevator as a non-option.
Remember with fitness than every bit counts. Just because you don't do push ups or sprints during lunch doesn't mean your body still isn't benefiting from the movement.