This is one of my favorite non-contact drills to do with my players during the season. It not only is a great conditioner but it also forces creativity and allows them to work on shots THEY want to work on. Once the player reaches 21, they must make five free-throws before getting water. Repeat four more times to simulate four quarters (even college players).
I prefer the players limit their lay-ups to 5 or less per round. If they are not great shooters, allow them work on their floater/runner game.
To make the drill more competitive and measure improvement, keep track of the time it takes each round and compare throughout the season.