Stir the pot
This is a bit more of an obscure ab exercise, but is one of the best out there for challenging the entire core at once.
Stir the pot looks kind of like a plank done with your forearms on an exercise ball. In fact, that’s exactly how you set yourself up. The challenge is the actual stirring -- your job is to make circles with your forearms, moving the ball in the process.
You should find that your abs hate you, your obliques want to cry, and your lower back is screaming in agony; in a good way, of course.
Like always, it's about the quality, especially since in the beginning, balance is going to be half the battle. I suggest people try out three sets with five circles going each direction, three times a week.