Here's a fun activity for groups. It combines match play with an element of surprise. I found it in an evolve9 activity booklet. You will need two cones and some clothes pins.
Divide players into two teams. Place cones on either side of net post off court. Have each team line up behind one of the cones. Players take turns playing singles against each other in order as they are lined up. Whoever wins the point places a clothes pin under their team's cone and goes to the end of their team's line. Whichever team has the most pins at the end of the event is the winner.