Today I had the bright idea to combine a balance board with an ab roller to do an ab exercise. You put your knees on the balance board then do the ab roller like normal with the goal of keeping the edges of the balance board off the ground.
THIS IS INTENSE. I can't think of an ab exercise I've done that's more difficult. I'm not yet able to do it very fast and it requires a lot of concentration. I imagine I will get better at it. I do not know anything about the physiological benefits of this exercise so I don't offer any official endorsement for you to do it, but if you have a chance to try this, I say to give it a shot.