You can try stretching while at your desk. Stretch your arms, legs, neck and torso while sitting. This will help prevent you from feeling stiff. You can also try stretching your shoulder, arms, wrists, ankles, calfs, chest and abdomen. All such stretching will help you out burning more calories and will keep you fit at work.
Look for opportunities to stand. You'll burn more calories standing than sitting. Stand while talking on the phone. Eat Lunch standing up.
There is much more you can do at your desk. Try googling some tips and you will have plenty.