Group: Line the players up behind the service-line T. Each will start moving forward when you make contact with the feed. Feed so that they can meet the ball at full reach roughly five feet from the net. They should have time to move forward, turn, set up, and hit a well-balanced overhead.
Individual: Have the player start from the service-line T each time. Keep track of her personal bests for sets of ten feeds.
Scoring: Each player gets ten feeds, taken one per turn in line. If the ball lands in the singles court, then bounces two more times before hitting the fence, it's worth one point. If it bounces only once after landing in and before hitting the fence, it's worth three points. If it lands in and then hits the fence, with no bounces in between, it's worth five points below the crossbar (halfway up the fence) and seven points above. The players usually love to try to bounce balls over the fence, and the fun usually justifies rewarding such shots with ten points despite the questionable wisdom of trying such a shot in a match. (You have to aim shorter to bounce a ball over the fence, and you might end up either hitting the net or not producing enough power to put the ball out of reach.) The best accumulated score after ten feeds wins.
Difficulties: For your own safety, feed from well behind your baseline, especially to powerful players.
