That's really funny! I do crunches on the ball, it's easier on my back, but it still burns! I also do hip extensions. Lay on the floor with your ankles resting on the ball and raise your but up til you're right in line with your legs. I hate push ups, but you can do these too on the ball. Here's a site you might like...
http://ronjones.org/Coach&Train/Exe...icExercises.htm