I have a love/hate relationship with ab exercises using the stability ball...they are challenging but work so well!! (Tip: keep your abs flexed throughout the exercises to keep the back straight and pelvis in a good neutral position.) Here are a few great moves from Women's Health:
Stability Ball Roll-Out
Kneel in front of a stability ball with your knees hip-width apart, then place your forearms on the ball, hands in loose fists (a). Keeping your back flat, brace your core and slowly roll the ball away from you by straightening your arms; extend as far as you can without allowing your hips to drop (b). Pause, then bend your elbows to roll the ball back to start. Repeat eight to 10 times.
Stability Ball V-Pass
Lie faceup on the floor, holding a stability ball overhead with both hands, your legs together and extended straight on the floor (a). In one motion, brace your core and lift your arms and legs off the ground, placing the ball between your feet (b). Squeeze the ball with your legs and lower your arms and legs back to the floor (c). Repeat, passing the ball back to your hands. That's one rep. Do eight to 10.
Stability Ball Single-Leg Press
Rest your upper back on a stability ball and cross your left leg over your right knee (a). Lower your hips toward the floor (b). Pause, then press through your heel to return to start. That's one rep. Do eight to 12, then repeat on the other side.
Plank Shoulder Taps
Get into a pushup position with your hands shoulder-width apart on the floor and your shins resting on top of the ball (a). Keeping your hips square to the floor, lift your right hand and tap your left shoulder (b). Return to start and repeat with the other arm. Continue alternating for a total of 26 reps.
See the full article: http://www.womenshealthmag.com/fitness/stability-ball-roll-out?workout=33774