Basic Pursuits

The classic push up is a super convenient yet very comprehensive exercise. One can accomplish a lot in a short period of time, increase arm, shoulder, upper back and chest strength. Do it all right in your bedroom for free.

Check out this course for doing 100 push ups a day. Combine this with some pull ups and you've got an upper body workout that's as good as it gets without setting foot in a gym (not that I have anything against gyms). And you're done in under 20 minutes.

One Hundred Push Ups (via LifeHacker)