Let's assume there are 12 players at this training session (if the numbers are different you can adjust accordingly)..There is a grid with 3 zones.  The two outside zones are 20 yards wide and 15 yards long.  The middle zone is 20 yards wide and 5 yards long. 

Start with 3 Defenders in the middle zone and the other 9 players spread out in the two outside zones.  The 9 players have soccer balls at their feet.  See diagram below for set up

There are four 2-minute periods with a 30 second break between periods.  The objective is for the X's to go from one outside zone, through the middle zone to the other outside zone without losing possession of the ball.  Each time an X makes it all the way through the center zone with the ball, they get a point.  At the end of the 2 minute period, the coach adds up all points scored by the X's and then subtracts the number of times when the defenders won a ball and knock it back to the zone it came from.  This combined total is the number of points for the defensive team (the fewer points the better for the defense).  For each period, there are 3 new defenders.  At the end of game, the defensive group that allowed the fewest number of points wins.

This is a great game (especially for young players) as they work hard, have to make decisions (should they go across or wait for a better opportunity?) and defensively have to communicate and work together in order to be most effective.  Plus, it allows the players to engage in 1 v 1 play and learn how to deal with it both offensively and defensively.