One of the hardest and less sexy cardio devices on the planet is the jump rope. Don't laugh, I will have some of my better clients jump rope for 2 minutes straight between exercises. That is their rest time, so instead of chatting about the stuffed Oreos they ate or how much they drank at the Chiefs game, they jump rope and most of them have a very difficult time finishing the two or three minute rounds.
Sounds simple, but try this on for size.
Jump rope for 60 seconds then rest for 30 seconds and repeat for 15 minutes. IF you can actually complete it you are better than 95% of people who will ever walk into the gym and beat the boring old eliptical anyday.