The pushup and row works your back muscles just as hard as your chest, making it the perfect two-for-one move when you're short on time.
Place a a pair of dumbbells on the floor and set yourself up in pushup position, grasping the handles with each hand (You can go fairly heavy since you're rowing the weight, not curling it). Lower your body to the floor, pause, then push yourself back up.
Once you're back in the starting position, row the dumbbell in your right hand to the side of your chest, keeping your elbow close to your ribs. Pause, then lower the dumbbell back down and repeat with your left arm.
Tip: Stay parallel to the floor. Your torso will want to rotate as you row. Don't let it!