Tricep dips are one of those body weight exercises that's harder than it looks. With your positioning and the fact that you're doing a type of triceps pushup, you'll really engage the back of the arms or, what my grandmother calls your 'boing.'
Another bonus is that this move involves other muscle groups--the shoulders, abs and back all work to keep you stable and even your legs work to keep you in position.
Do it right: Begin sitting on a step or chair with hands next to thighs. Balance on your arms, moving your rear end in front of the step with legs straight (harder) or bent (easier). Bend the elbows and lower body a few inches, keeping the shoulders away from your ears and the elbows parallel to one another, going no lower than 90 degrees. Push back up to starting position. Repeat for 2-3 sets of 8-12 repetitions. If you have any wrist or shoulder problems, skip this exercise.