Cardio and a heart rate monitor...
I never use one because I am not concerned about hitting that supposed fat burning zone. The purpose of the interval training is to be above all of those other recommended moderate level cardio activities. You want to have those bursts of activity in your cardio sessions so that you raise the level of Growth Hormone in your body, hold onto more muscle mass, and burn fat 24 hours per day. When you do moderate level cardio in the "fat burning zone" you will burn more calories during that timeframe, but afterwards your metabolism slows down... Whereas with interval training you have revved up your metabolism and Growth Hormone release for the entire day making you burn more calories over a 24 hour period. Does that make sense? Now, you don't want to do the interval type training more than 3 days per week as you will begin to breakdown your body do to overstressing it. Cortisol levels raise too high when you do that. That's why when I recommend more cardio above and beyond the interval type cardio, I always recommend moderate level cardio. For that type of cardio you can use a heart monitor to keep you in the correct target range. 65-80% of your maximum heart rate is a good place to be for those sessions. Your maximum heart rate is approximately 220 - age.