Well, Pilates is supposed to be da bom for building core strength. I've done it, and it's good!
I've also done some workouts with those large inflateable balls (things like: you do pushups with the ball supporting your legs, the further down your body it is the harder/better it is, etc.) and those are good too, but I don't enjoy it so much--yes, as a beginner you can expect to fall off a few times. And it's a bad idea to laugh while doing anything on a ball--that shakes the very core muscles you're trying to work.
Those hovers you're doing are terrific for core strength...keep it up! How many times do you do a 60-sec. hold? My trainer recs. a set of 3. It's killer. Killer!