Once you've mastered squatting on the ground and then squatting on an unstable surface like a core board or Bosu, you can amp it up like a pro skier by taking it all the way to an exercise ball. (You'll definitely want a spotter!) This tricky move requires impeccable balance but activates every muscle in your body. Bonus: You'll be concentrating so hard that you won't even realize how hard you're working until you're sore the next day! Try for 3 squats—although even 1 rep is a major accomplishment!
Training tip: Did I mention having a spotter? Get a spotter. Or two. Make sure they stand close enough to grab you if you start to fall. If you're at all unsure of your balance, skip this move.