Dirty Harry

What this has to do with
Dirty Harry I do not know

Here's a fun activity to improve lateral movement and quickness. Coach is on one side of net on one half of court (ad or deuce). Players are across net straight ahead of Coach. For this example, let's put the Coach at the net on the ad side, and players across net at deuce side baseline. Coach tosses the ball underhand diagonally to the ad side baseline. Player must catch ball after bounce, moving laterally with good quality side shuffle. After retrieving the ball, player shuffles back to starting point, tosses ball back to Coach, and returns to end of player line. One point is awarded for each ball caught. First player to earn 7 points calls out 'Dirty Harry' and wins the game.

Move to opposite side of court and repeat so that players work on moving laterally from both sides of court.

Progression
The version above is the hardest. To make it a little easier, consider these tweaks.
Easy - coach is on same side of net as players; uses smaller court or only half of court
A little harder - Coach is across net, but allows multiple bounces.

Update: I tried this one recently and it worked great! I had the students warm up by starting on the deuce side with catching the tossed ball, with me tossing from the same side of the court. First one to 7 points wins; repeat on ad side. Points only awarded if they shuffled over, caught ball after one bounce, and shuffled BACK. Then we progressed to them shuffling over, hitting ball, shuffling back. I was feeding from across the net. No points awarded unless 1) ball went over net into play; and 2) shuffle over AND BACK. I fed quickly, so they were huffing and puffing after four rotations. I will definitely be using this again.

Adapted from USTA's Learn To Rally And Play