A simple exercise you can do anywhere at any time is as follows:
Either standing or sitting, keeping both feet together, raise your arms into the air above your head as high as you can with palms facing each other.  Hold this position as long as you can.

When comfortable with this position you can try stretching your palms so that they face upwards. Try alternating between keeping the fingers together and apart. This is a good stretch for the tendons in the wrist and elbow which are prone to repetative stress injury.