Strength building exercises you can do using only your body weight as resistance.

Pull Up

Body Weight Exercises for Strength
Grip bar with hands slightly wider than shoulder width. Keep body in good alignment (abs tight, chest up, back straight). Squeeze down (depress) shoulder blades using a controlled motion, pull body up by drawing chest to the bar, keeping elbows down toward sides and forearms in line with body. Contract lat muscles fully without compromising form. While maintaining the controlled motion, return body to starting position. Do not allow body to swing. Do not allow muscles to relax before next repetition.

Back Extension

Body Weight Exercises for Strength
Lock feet in, with pad comfortably on hips. Cross arms over chest. If using weight, hold weight plate at chest level. Pressing hips into the pad, relax the spine. Squeeze together (retract) shoulder blades. In a controlled motion, lift head and shoulders up until spine is comfortably hyperextended. Avoid hyperextending the neck. Maintaining controlled motion, return to starting position. Do not allow muscles to relax before next repetition.

Mountain Climbers

Body Weight Exercises for Strength
Primary Muscle Group: Full Body

Muscle Groups Worked in This Exercise:  Full Body

Preparation: Hands on floor with shoulders over hands.  One leg tucked, one leg extended.  Weight should be placed on balls of feet.

Breathing:  Breath out on the contraction of the muscle and breath back in on the way back to the start position.

Execution: As quickly as possible "climb" alternating legs back and forth.

Back Extension

Body Weight Exercises for Strength
Lock feet in, with pad comfortably on hips. Cross arms over chest. If using weight, hold weight plate at chest level. Pressing hips into the pad, relax the spine. Squeeze together (retract) shoulder blades. In a controlled motion, lift head and shoulders up until spine is comfortably hyperextended. Avoid hyperextending the neck. Maintaining controlled motion, return to starting position. Do not allow muscles to relax before next repetition.

Push Ups Explosive

Body Weight Exercises for Strength
Primary Muscle Group: Plyometrics - Upper Body

Muscle Groups Worked in This Exercise: Chest

Preparation: Start in a standard pushup position.

Execution: Lower yourself to the ground and then explosively push up so that your hands leave the ground. Catch your fall you're your hands and immediately lower yourself into a pushup again and repeat.