Hindu pushups

Hindu pushups are great. They work whole upper body and also give you good cardio workout. Here's short description how to do them by Mike Mahler:

Hindu push ups
This is a kickass exercise and one of my favorites. In addition to being a great upper body exercise, it's great for increasing flexibility in the spine and is a fantastic pick-me-up whenever you feel lethargic. Get on all fours and press your butt in the air. Bend your back as if you were trying to make a triangle. Your head should be aligned with your back facing towards your feet. Take a deep breath and then sweep down in a circular arc motion and bend back looking up at the ceiling and breathing out. From there, push back toward your heels and start over.