Depth Charge appeared in an article from Tennis Pro magazine (December 2010) by Rob Antoun. It was suggested as a warm-up, but I love it for working on consistency and depth any time during the lesson. You will need some way to mark the back court in half sideline to sideline.
Players pair up to play singles. If you have an odd number, the odd player out stands at the net post to keep score. If numbers are even, one of the two singles players is keeping score as well as serving as a cooperative rallying partner. The non-scorekeeping player has one minute to accumulate as many points as possible as follows:
1 point for ball hit over net and into service box
2 points if ball lands in front half of back court
3 points if ball lands in back half of back court
Rotate until each player has had their turn. Player with most points at end of last rotation wins.
Too hard? use a smaller court or slower balls, and allow 1 point for in front of service box, 2 points for behind service box. For your youngest beginners, have them toss and catch instead of rally with racquets.
Too easy (or need more courts)? Shrink court in half lengthwise baseline to baseline and have four players playing two rounds of singles straight ahead simultaneously. Also consider subtracting points if balls are hit out or in net.