Always opt for stairs over an elevator, when possible.
If your office has low-traffic, easily accessible stairs, set an alarm and every two or three hours spend a few minutes doing an interval workout. It's easy to do because it requires no equipment. You might want to consider doing this in your socks, however, depending on what shoes you're wearing.
Here's how to do it: Sprint up one flight of stairs as fast as you can and then walk down to catch your breath. Repeat this five times — you'll be surprised at how fast this gets your heart rate up. If you want to intensify this even further, after doing those intervals, Catudal suggests jumping up a few steps at a time with both legs and then jumping back down and squatting 10 times for a total of three sets.