Build muscle and blast calories with this body-weight workout. And it's excuse-proof, too. You can do it anywhere since it requires no equipment at all. The workout should take you around 20 minutes, so get moving!
Directions: Warm up with five minutes of light cardio, then repeat each three-exercise circuit three times. Start with 10 reps of each exercise, building up to 15 reps of each move as you get stronger.
Here's a printable version of this workout. Now you really have no excuses!