I do high intensity intervals on one of three machines
Usually it's the elliptical, but there are some nights when I look at that fucking thing and can't bear the thought of being on it.
It's usually 25-30 minutes and it breaks down like this:
First 5 minutes, warm up at about 110-115 steps/minute
Next 10 minutes, 30 seconds balls out with a minute of sustained pace at about 110 steps/minute.
Next 5 minutes, 1 minute balls out, one minute 100 steps/minute
Last 5 minutes, 30 seconds balls out, one minute at about 100 steps/minute.
Then for that last 5 minute "cool down" I'll stay on it nice and easy until my HR gets down to around 120-ish.
I definitely like the ideas of jumping rope and high rep squats as an alternative, I've also thought of doing 90 second sets hitting the heavy bag. For me, my biggest enemy with cardio is boredom and monotony, so any new ideas help a lot.