White Bear Racquet and Swim in White Bear Lake, MN, which I believe has since been acquired by the Life Time Fitness chain.
Since I learned it as a doubles player, let's use that as our example. Four players take the court. Extra players are waiting at the net post. Three of the players on court have two balls each tucked away in pockets/skirts. The fourth player has three balls. Whomever has three balls feeds the first ball, bounce or underhand feed (not serve). Play out the point. Player making error then feeds the next ball. Repeat. Whenever someone runs out of balls, they leave the court and the next player waiting comes in. Be sure this new player has three balls and starts the next point.
You have probably figured out that this is a great drill to emphasize consistency. The players who make the fewest mistakes stay on court the longest. For young players/beginners, make sure they hustle on and off the court. If they can't rally much, at least they should get a good workout running on and off the court! For my young players this also has a secondary benefit: reinforcing the importance of wearing clothing that has pockets or that can easily store at least two tennis balls.