I have fallen in love with an exercise that is very simple yet extremely challenging. It works almost every muscle in your body simultaneously and will literally leave you shaking.
First find a decently long staircase 20 steps or more is ideal. apartments, stadiums, schools, gyms, or any other public buildings will be the best places to find such a staircase. Put your hands on the bottom stair or on the landing of where you you are starting. Put your feet several stairs above your hands (just how many stairs depends on your height and the angle of the staircase.) Now you are essentially in an incline pushup position. Now start by moving one hand up a step or two then do the same with the other. Almost at the same time you are moving your feet up one or two stairs at a time. The 2 key points during the exercise are: always keep your body in as straight a line as possible; your hips may be slightly higher but you should be able to draw a straight line from your heels to your head. The second point is that throughout the entire exercise only your hands and feet ar touching the staircase. Not your hips, legs, or chest, etc.
This exercise is only made better by alternating it with stair sprints. Now you have some great fat burning cardio coupled with a devastating strength exercise. Usually 10 sets of stair sprints per one set of stair crawls. Repeat this 4-5x and you just completed one of the best workouts of your life. If you are sadistic try doing the stair crawls going down the stairs. You think It'd be easier but it's not. Enjoy!