Treadmills, elliptical machines, and stationary bikes all offer great cardiovascular activity, but they can also get boring very quickly. Interval training is the perfect solution to keep your interest. Interval training alternates a short burst of high-intensity exercise with bouts of low-intensity activity, burning more fat in 20 minutes than longer workouts do.
Here's how to do it: Warm up for five or 10 minutes on a stationary bike, treadmill, or on a run. Then pedal, walk, or run as fast as you can, for 20 to 30 seconds, followed by a minute or two of low-intensity activity. Speed up again, then lay back. Do five or six alternations in 20 minutes.