I will be posting some cooperative activities modified for tennis, inspired by a recent post on ThePhysicalEducator.com. The first is Back Me Up. It works great as a warm-up. Take care to work safely as they will be working on a hard surface (the court).
Create two-person teams of players evenly matched for size. Have them sit on the court with their backs together. Knees should be drawn up to chest. Challenge them to achieve a standing position by using the pressure they apply back-to-back.
If they are able, have them try getting back to a sitting position using the same pressure and balance skills.