I've been doing HIIT on the elliptical, it's easy to vary your intensity quickly. I usually do 30 seconds as fast as I can, and then slow and recover for 45 seconds.

I can do 16 of these cycles in a 20 minutes session. I 've been doing it for about 8 weeks now and have lost about a pound a week, but 3 inches around the waist. I do one session daily.