Muscles worked: abs, obliques, quads, hamstrings, deltoids, biceps, triceps, and the chest.
Start off in the standard push up position. Pull your knees up to your chest, one at a time, in quick succession. You want to keep your body in a push-up position all the way through the exercise, so don't bob your hips up and down as you're tucking your legs in.  Do 20 seconds three times.

Muscles worked: hamstrings, chest, triceps.
Start by standing up, legs a little bit less than shoulder-width apart. Bend over like you wanted to touch your toes, but instead, place your palms on the floor and walk forward with your arms until you reach the standard push up position. Do a push up then walk back with your arms and straighten back up. Do 3 sets of 10 reps.

Muscles worked: quads, glutes, calves, hamstring, core, inner thighs.
As you are standing straight, take a big step forward and bend your knee so your rear knee almost touches the floor. Then kick yourself back up with your front leg and return to your original position. Do the same with the other leg. Once you did both legs, that's one rep. Do 4 sets of 12 reps.

Muscle areas activated: triceps, delts, pecs, upper back.
Box dips can be performed almost everywhere where there is an elevated surface (a chair, a sofa, a low wall, etc.): Rest your legs in front of you as you do your dips and concentrate on slow negative movement (as you go down) and explosive upward motion. Do 3 sets of 8 reps.

Muscle areas activated: all your back muscles, biceps, forearms, core.
You need is a surface you can hang from with your legs extended in front of you. Start by hanging onto the bar, heels on the floor, your back and legs straight, core engaged. Pull your body closer to the bar then straighten your arm again. The negative movement (as straighten your arms) should be slow and the positive very energetic. Do 3 sets of 8 reps.