The box game is one of my favorite drills because it can be adjusted for different levels of play.
How the drill works: Make a box using whatever is at your disposal (cones, balls, etc.) in the deep corner of the deuce side. The size of the box can vary from 2' x 2' for professionals to 6'x 6' or even bigger for lower levels of play. Player A stands in the center of the court on the opposite side of the box while player B stands behind box. The objective of the drill is for player A to hit the ball into the box as many times as possible. It is player B's responsibility to keep the ball in play to player A mixing up forehands and backhands but not so difficult as to make it impossible for player A to hit the target.
Scoring: Starting from 0, every time player A hits the ball into the box their score goes up one. Every time they miss the box the score goes down one. Player A must keep playing until he or she reaches 20. If player B hits the ball out or hits a very difficult ball to player A, this is a "free ball". Which means if player A makes it in the box they get a point but if they don't it doesn't count. This is important so that player A tries for every ball, not just the easy ones.
Variations: This drill can be adjusted for the level of play. The most important thing is for the drill to be challenging but not impossible for the player. For lower levels increase the size of the box and play up to 10 or 15 instead of 20. For higher levels, make the box smaller and play to 20 or 25. After completing the goal on the deuce side, move the box to the ad corner and try again.
Areas of Focus: The great thing about this drill is that in order to be successful you must be focused mentally. You will find the better you become at this drill the longer you will be able to keep your concentration during tough points in match play. Your unforced errors will begin to decrease dramatically. You must also move your feet and have good balance to be able to hit the target consistently. If done right, this drill is a fantastic cardiovascular workout.