Some suggestions to try might be various types of yoga exercises done while seated at your desk.
While seated at the front edge of your chair with your feet flat on the floor and knees directly over your ankles, you could try neck rolls by gently dropping your head forward and rolling from side to side. You could also try shoulder rolls by gently lifting your shoulders and rotating them backwards in slow circular motions. Repeat a few times and then reverse the motion. One more exercise to try is moving your arms behind you until you can hold onto the back of your chair. You should keep your elbows straight and adjust the height of your hands until you feel a good stretch in your chest, arms and shoulders.