This is just a variation on a regular rectus abdominus crunch. If you have access to a bunch of small beach balls, bring them out for this one. Have everyone lie on their backs and put the ball between their knees. Then, hands at temples, eyes focused on the ceiling, curl up and down. For greater intensity, lift feet off the ground (hips and knees bent 90 degrees). For greater intensity, put ball between feet. This is a regular crunch but when you add the ball, inner thigh must work to squeeze and hold it in place. Also, balls are lots of fun!