Chin-ups! If you even do two or three you'll impress all the boys. When you've done as many as you can, do a couple-three more eccentric (climb up to the chin bar, then let yourself down slo-o-o-o-wly, 30 sec if you can), that's how you build up to doing more full ones. Or your gym may have an assist machine where you kneel on a cantilevered platform.