I helped teaching a kids' class for a while, and we used to play a game we called ukemi-catch. It's simply regular catch except that you cannot be caught if you are performing some sort of ukemi. A lot of fun and VERY good at building stamina.
I also used to help teaching a beginners class in Karate and since we had far to few showers we used to end practice with small contests - each contest resulting in the winner being send to the showers. The contest could be the first person to touch the wall in one end of the dojo, the first one to do 20 push-up's, the first person get down on the back or the first person to stand on his head. I guess you've got the picture by now. It's a great way to end practice because everybody has fun and it exercises the students alertness.