I am a huge fan of push-ups since they strengthen your upper body and core beautifully, and they require no equipment whatsoever. Regular push-ups can be tough to do all in a row (and pretty boring too), so here's a variation that lets your upper body stretch in between push-ups.

• Begin on you hands and knees, with your hands shoulder-width apart, and your knees underneath your hips.
• Keep you arms straight as you straighten out your legs behind you, coming into plank position.
• As you inhale, bend your elbows as you lower yourself down into a push-up (with control), keeping your body parallel to the ground.
• Exhale to straighten your arms and lift your right arm straight up over head, making a T-shape with your body.
• Inhale to place your right palm back on the ground and lower into a push-up.
• Exhale to straighten your arms and lift your left arm up overhead.
• Inhale to place your left hand back on the ground and lower into a push-up.
• Continue this exercise, repeating a equal number of times on both sides. Try to do at least five on each side. As you get stronger, you can work your way up to 10 on each side.