Bellydance!
I’ve been dancing for two years now, but I developed what has been called (by an outside source) “rockin’ abs” in not too very long (that’s abs in the tight, non-six pack sense—you still look like a chick!... unless you’re a guy. Anyhow.). It’s a muscle-based dance, and the basic moves can be learned relatively easily out of a book (I recommend Belly dancing basics by Laura Cooper, which I used at the beginning). Hip isolations, for instance, begin by using the knees to power them, but after you’ve got the basic principle down, you begin to manipulate using mostly—or only—your obliques, and most of the other moves are largely controlled by the stomach muscles. Plus it works out a lot of the rest of your body (arms and back, particularly) and is amazing fun!