This is what I did today....
Went to the rock climbing gym and basically tried to get up the 25 foot wall as fast as possible. Jumping and skipping holds and just grabbing what you can to get to the top. I did this about 15 times in a row. My lungs were burning for a half hour after that. I was in pain. That was the best cardio that I've done in a long time. People were just staring at me like I was crazy.
Down side is, I ripped a chunk of skin off at one of my calouses. That hurts. I'm gonna have to hold off on doing that again for a while.
If you can, try it. AWESOME cardio!!!