Cardio is the one area of fitness where the fun stuff actually works better than not-fun stuff. For one thing, you'll work harder, without realizing it, when you're having fun or you're at least mentally engaged. A few studies back this up... as if you need studies to know that. Also, cardio for fat loss is all about being inefficient. If you get efficient – really good at one form of activity – you'll burn less fat doing it. So cardio for fat loss really is where you want to "keep the body guessing." You don't want to get too good at any one form of cardio. If you do that, then you'll have to do MORE of it. And who wants to do more cardio? So, variety and unfamiliar activities work best for metabolic conditioning.

I call one of my favorite inefficient cardio workouts "20 minutes of anything." Set a timer for 20 minutes (or whatever) and jump from one activity to the next until the timer goes off. Since I have access to the T Nation Training Lab, I can choose Prowler and sled pushes and pulls, banging tires with a weighted club, battle ropes, kettlebells, etc. I usually pick two of these, toss some ab work between them, and keep going until the iPhone timer bings.

I also used to train at a gym that was closed on Sundays. I'd take a suspension TRX-style device to the park along with a medball. The workout was: Throw the ball behind me as hard as possible (scoop toss). Sprint to the ball and do it again. Repeat until I couldn't catch my breath. Walk to the TRX and do push-ups, rows, and pull-ups. Go back to the medball tosses and repeat until the timer beeped.

At a commercial gym you're only limited by your imagination, but try to pick things you're not very efficient at. If you can kick ass on the rower, use the stairmill instead. If you get really efficient at stairmilling, don't do more of it, just switch to a new machine. Heck, you could choose four cardio machines and do about 6 hard minutes on each one. As a bonus, the countdown clock provides some "positive pressure" and helps to keep you moving.