Web Tennis Drills. When I first saw the name I thought it would be an attempt to rally 100 times. It is, but it isn't. Two players rally, counting the number of rallies as they play. When someone misses, the other player gets the number of points they had played to. So for example if they hit 12 balls and player A misses ball 13, Player B gets 12 points and they resume. First player to get 100 points wins.
This activity easily qualifies for my Frequently Used list. It is simple, it has consistency at its core, and it is easily adapted to any level player. Players can't rally yet? Let them toss underhanded. Too easy? Have them hit only forehands, or only volleys, or only cross-court. This activity would easily work for doubles or as a mini-tennis doubles warm-up. Perfect also for a team warm-up rather than that silly useless league warm-up everyone does these days. 100 takes too long? Surprise your players by having them select a number somewhat at random ("pick a number between 25 and 75"), and that will be the number of rallies needed to win.
Update: I used this activity yesterday with an odd number of students. At first I had them playing 2 vs 1, rotating frequently so no one was stuck as the Lone Ranger for too long. This does not work. The side with 2 players invariably skewed toward one player playing most of the balls and if there is an error made, that player gets the points. In fact one player figured this out and started hogging balls just so that if he forced an error, he got the points. So I switched it up so that no matter the number of players per side, only one was playing out the point, and rotate players after every point. Problem solved. We went with the number 38 rather than 100 due to a random choice by one of the players. These were 10U beginners and it took them more than 20 minutes to finish. Whew!