I'm discovering that if you simply do *regular* ab workouts on the Bosu, they suddenly become super advanced! I like the Bosu much better for this than the stability ball for some reason. Just do crunches (slow, and really let your upper body fall back over the ball to stretch out the abs, and while lifting, keep your shoulders retracted back and lift completely with your abs - do 10 of these and your abs will be on fire!), oblique crunches, side lying oblique work with hips stacked on Bosu. Then, if you have these mastered, try doing bicycles, in and outs, leg lifts (but never let your feet hit the floor - balance with lower back on top of Bosu, and extend both upper and lower body so you are a board - this alone is tough. Then slowly raise both feet as far as you can, holding your upper body balanced straight, then slowly lower the legs back to parallel. Wicked.), v-sits....
Then flip the Bosu over so the curve is on the floor, and do planks holding the platform. Do all the plank variations that Cathe does - lift one leg, lift one leg and sweep it out to the side, bring one knee into your chest, etc.
It makes anything you do on the floor 10x more difficult if you do it on the Bosu. I'm loving mine! ARX, Ab Hits and Core Max will never be the same!
m.