Dribbler

This game is harder than it seems. It is easily adaptable for different size groups and abilities.

Divide players into two groups at one of the baselines. Have a set number of balls for each team. One at a time, players dribble a tennis ball with their racquets up to the service line and hit it over the net. Note they must dribble continuously and hit the ball over the net without stopping the dribbling - the hit over the net must be off a dribbled ball.  If the ball does not go over the net, player must return to baseline and repeat attempt. First team to hit all of their balls over the net and in play wins.

Variation
For small groups, play as individuals rather than as teams.

Progressions
Easiest Have players dribble by hand with the 'bounce, catch' and toss ball over net.
Harder Divide target court into sections, awarding varying numbers of points. For example, ball landing in the service box is one point. Past the service line is two points. No Man's Land or deeper is three points. Team with most points when all balls are played wins. Another option: team's ball must land in team's half of opposite court.
Hardest Smaller, more specific targets are the only way players can earn points. For example, place hula hoops or cone circles at various positions on the court. Ball must land in the target to earn a point.

Adapted from The Tennis Drills Book by Tina Hoskins.

Update: I was astonished at the level of cheating during this activity. Things to watch for:

• Players carrying multiple balls to the service line. 
• Players ignoring/not replaying their net balls. 
• Players hand-feeding or otherwise handling the ball with their hands.