I added those plyo pushups on a med ball a few months ago, and they are by far my favorite. If you want to do an ab exercise that is very similar, but not as hard you can try these, I think they're called Swiss ball body pulls. Start in a plank position arms straight, with your ankles on the ball, push back and roll the ball up to your thighs and touch your head to the ground while still keeping your arms straight in front of you. Now, roll back to the original position, but this time keep going and roll into a pike position with your butt up in the air. Then roll back and touch your head again. It really works the abs, but not as torturous as that little evil wheel.