Calisthenics
Calisthenics are exercises consisting of a variety of simple movements, usually performed without weights or equipment, that are intended to increase body strength and flexibility using the weight of one's own body for resistance. Repeated motions of calisthenics done over an extended period of time builds muscle endurance.

The primary calisthenic exercises, I use, are:

Push-ups

Start face down on floor, palms against floor under shoulders, toes curled up against floor. Push up with arms keeping a straight line from head through toes. Lower again, to a few inches off floor and repeat. You should keep your head tilted upward, your back straight. Do not rest on your shoulder blades, even when you feel fatigue. This trains your chest, shoulder, and tricep muscles. The types of push-ups can vary through the different positions you can place the hands.  Regular push-up will have your fingers pointing forwards, allowing the exercise to focus on the center of your ch est.  Wide arm push-ups, or hands facing out from your body, will focus the exercise on the pectoral tendon towards the outer areas of the chest and also allows your chest to have a nice wide stretch. The last hand position would be having your hands turned as comfortably backwards.  This will focus the exercise on the biceps and again the center of the chest. Push-ups can be modified by performing them on your fists and inclining your body.

Sit-ups

Start with your back on the floor, knees bent, bottoms of feet against the floor. Lift shoulders off the floor by tightening abdominal muscles bringing your chest closer to your knees. Lower back to the floor with a smooth movement. This trains your abdominal muscles.

Crunches

Like the sit-up, except instead of bringing the whole torso area closer to the knees, only a concentrated but shorter movement of the abdominals is performed. Shoulder blades are lifted off the floor, and abdominals tightened.

Squats

Stand with feet shoulder width apart. Squat as far as possible bringing your arms forward parallel to the floor. Return to standing position. Repeat. If this is not challenging, you can do variations. One variation is lifting one leg off the floor in front of you, putting both arms in front of you for balance, and squatting. This is a one-legged squat or pistol. Squats train the quadriceps, hamstrings, calves, and gluteals.

Calf-raises

Stand on a platform with an edge where you can let the heels hang. Use your heels to lift your body on the balls of your feet, then slowly return to starting position. This trains the gastrocnemius. A seated calf-raise trains the soleus.

Dips

Jump up into position in between parallel bars or facing either direction of trapezoid bars found in some gyms. Cross your feet with either foot in front and lower yourself on the bars until your elbows are in line with your shoulders. Push yourself up until you are fully extended, but you do not have to lock your elbows. Press yourself up and down for repetitions and you are doing dips. Dips can also be done by sitting on a chair and extending out your legs. The legs do not have to be locked out, but can have a bend to them.  Push yourself up and position yourself so that you can raise and lower your upper body away from the chair.  These dips can also be called tricep push-ups. Dips focus primarily on the chest, triceps, and deltoids.

Pull-ups

Start by grabbing an overhead bar (called a chin-up bar) using a shoulder-width overhand (palms facing forward) grip. Keep your back straight throughout. Using your lat muscles, pull yourself up to chin level (always with the bar in front of your head) then slowly return to starting position in a slow controlled manner. Avoid using the arms to pull yourself up and do not make jerky movements to gain leverage. This primarily trains your lats or upper back muscles, as well as the forearms. An underhand grip variation or chin-up trains both the back and biceps.

Strategy

I like to do a 20/20 (20 push-ups and 20 sit-ups) combination between drills. The push-ups and sit-ups can be done in a variety of different ways. Here are variations of push-ups; wide arm push-ups, hands facing backwards push-ups, triangle push-ups, and elevated (or inclined) push-ups. Some variations of sit-ups or abdominal exercises are, leg lifts, alternating leg lifts or butterfly kicks, crunches, bicycle kicks, and partner leg lifts.