Here is one for you.Assume the top position of a push-up but with your feet resting on the stability ball.Take your left foot off the ball and twisting at the waist, push your leg underneath you until it is extended out as straight as possible but do not let it touch the floor.Now twist in the opposite direction bringing your foot back underneath you to return to it's original position.Do the same thing with your right foot now.Keep alternating reps.
These really hit my obliques hard and take some getting used to due to the instability factor.