Tabata is a form of HIIT (high-intensity interval training) that'll challenge you like nothing else: 20 seconds of work, 10 seconds of rest, and repeat until you're exhausted. Just 30 minutes long, this high-impact routine features air jacks (jumping jacks, but harder), push-ups, and burpees — and that's just the first 10 minutes! You'll need a sweat towel for this one.