Bunny, Thai Boxing and sparring seriously has to be the best cardio for both conditioning and calorie burning.
With an actual sparring partner (or just someone to hold up the pads),3 minute rounds with a 1 minute rest is incredibly tough. Kinda like HIIT only more intense I think. After 6 rounds, just holding up your arms can be a challenge. Wrestling, BJJ or any type of grappling is really tough too. I've seen people get shredded just doing Martial Art training and weights. Beats the elliptical any day. (God I HATE that thing.)