Don't forget stairs
I find stairs to be a complete cardio workout. If you are on a machine like Stiarmaster, try not to hold the bars. This will force you to tighten your core muscles as you control your equilibrium. Even better is to get into a local football stadium or arena and run the real thing. That will get your heart-a-pounding and sweat flowing. The longer/higher, the better. Running down stairs is beneficial too. On the way down you need to control your weight and momentum as you coordinate your steps. Awfully rewarding after a good 45-50minute workout.