Tabata The Hard Way.
8 Rounds of 20 seconds on, 10 seconds off. Stick with the same exercise for all 8 rounds. After, rest 1 minute and move to the next exercise. During the rest, do an isometric hold. This one gets tough pretty quickly. Stick with it and push through the pain!
Squats - Hold a squat during the 10 seconds of rest
Situps - Hold the hollow position during the rest
Pushups - Hold the top of a pushup during the rest