This simple but effective drill idea is from the 3rd Toad tennis blog. Its purpose is to get players comfortable with coming in to the net. Works for both singles and doubles. Also great for fitness and footwork.
Two players rally from the baseline cooperatively. After the third rally, one player moves in to the net and point is played out competitively.
For more than two players, divide players into more or less equal groups and have them rotate in after every point. Continue until all players have had a chance at both positions.
Variation: baseline player who wins point trades places with net player.