Distribute your players among your courts. Best if your courts are next to each other to speed up court rotation times. Extra players are stationed at the ends of the courts. This screen shot pertains to a school gym but it translates easily to tennis courts IMO.
At timed intervals, Coach stops play and calls a rotation. All players move one position to left or right depending on Coach's instructions. Every rotation should move one of the excess players at the ends onto court, so no one is left standing out too long.
Players play out points. At the end of each rotation, player with most points won gets one sticker or clothes pin to represent that he/she 'won' during that rotation. At end of activity, player with most stickers/pins, wins.