This is very basic, but it does the trick in getting the heart rate up. Just make sure they are using their arms. It is called "The Countdown" because that is precisely what you do. It is 32 count self-reversing. I generally start with ham curls, but you can use knee lifts, glute lifts, front kicks and hip abduction.
This is a simple traveling or corner to corner movement.
Then start from the top with left leading.