This is the second idea I am stealing from Valentina Moseychuk's junior tennis warm-up video. She did not have a name for it so I am taking liberties.
Place piles of 3 balls each along one sideline, one pile per player. Players compete to be the first player to move all three balls over to far sideline and back. Balls must be moved one at a time. Balls must be set down on far sideline one at a time. Player may not begin moving them from far sideline to original sideline until all three are at far sideline. In other words they cannot carry one ball over to far sideline and bring it right back. So they should be making at total of 6 trips across court and back. Click on the linnk above and watch the video at about :38 to see this drill in action.