After whiplash from a car accident resulting in chronic headaches, l spent some years with a physical therapist. They are a wealth of information. I learned exercises that can be done while driving, in a movie theatre or concert, in classes in school, on an airliner, or when sitting in church!
Following are some of them. Also check out the thread, "Deskercise" in this Fitness section.
1) Squeeze your butt/cheeks. No one knows you are doing it!
2) roll your shoulders forward and backward 20 times.
3) shrug your shoulders up and down
4) Roll your ankle around in circles to stretch it. Then, try to make the alphabet from your ankle (A, B, C,...etc)
5) squeeze your fingers together tightlty, then stretch them out tightly
6) pull your shoulder blades back together, then pretend you are trying to get them into your back pocket. Hold for 30 seconds and repeat 20 times. This strengthens your back to help improve posture.
7) tighten the back of your neck by pulling your neck backward. Hold for 30 - 60 seconds, repeat 20 times
8) stretch your neck from side to side
9) As often as you can remember, sit and stand with good posture with your ears over your shoulders over your hips.
Does anyone else have any other ideas?