Gotcha! is an easy and fun tennis warm-up idea from one of my co-workers, Coach Tricia. Tricia is a retired PE teacher and has many great ideas for small and large group activities.
You will need some bean bags or koosh balls or other soft items that do not bounce or roll, enough for one each. Restrict the game area to a portion of the court based on the size of your group. You want it small enough to make it fun, but large enough to be safe. In this example we were using the service box.
Play is during a timed interval. When Coach says Go, players try to tag each other on the foot with their bean bags. When they are successful, they call out 'Gotcha!' At the end of the timed interval, player with most Gotchas wins.