I'll second (or third or fourth) the swimming, but make it kickboarding, not swimming. Sure, do some swimming too cos that's great for all over toning, but if you want to work on your thighs with the added bonus of your bum, then only do 0.5km or so swimming and do another 1km with just your legs kicking. Best thing I ever found for bums and thighs. As an added bonus, you may find the core muscle strength you build through swimming helps with your hip issues.