Sometimes, it's hard to even find the time to get out of your chair. Don't worry: You can still get exercise!
One area to focus on is your upper back and shoulder. You can do twists or shoulder rolls without standing for an easy workout. Depending on how much room you have where you're sitting, you could also try rotating your arms or flexing for extra upper body strength.
The other area to think about is your legs. It may not be as direct as the upper body twists, but doing some simple leg crunches and stretches can be a perfect under-the-desk routine. Just make sure you also take breaks and stand up every once and a while, too! As helpful as these exercises are, it's also good to get up and get moving.