Jog Ball is a quick and easy warm-up for all ages and abilities. Players jog lightly in a circle. While they are jogging, they toss a ball up, let it bounce, and catch with both hands. Progress to catching with just the right hand, then just the left hand, then alternate. Once they are used to this, coach can have them change direction on command or using a whistle. Note the initial directions for this activity were not clear whether all players are in one big circle, or are each jogging in their own individual circle. For safety reasons, I recommend the latter.
Adapted from USTA's QuickStart Tennis guide