Jump rope.

If you haven't tried jumping rope lately, now is the time to give this fun and invigorating exercise a try. Purchase a jump rope or fashion your own from any sturdy piece of rope. Start slowly and gradually increase both your speed and intensity. This heart-healthy exercise can be performed a few times a day for maximum exercise benefits.

The first time I tried it, I could only last about 30 seconds before getting completely winded. It's definitely a workout.