I would choose a piece of rope and my one exercise would be to twirl it over my head and jump through the loop. There is no other single exercise that rivals the simple jump rope for efficiency when you take into account the complete fitness benefits it offers. Almost everyone has skipped rope at some time in his or her life, but other than boxers, most of us left it on the elementary school playground. Skipping rope for 15 to 20 minutes a day is practically unrivaled as a total body workout. Not only is it a good CV workout, but it improves your agility, fluidity, coordination, lateral movement, explosiveness, speed, and timing.
Boxers are some of the fittest athletes on the planet and skipping rope is their staple exercise.
While it obviously works out the legs, it also tones the entire upper torso - chest, back, shoulders, triceps, biceps, and forearms. The other nice thing is that you need neither much space nor equipment to exercise. Nor is it difficult to learn. If you have no experience at all jumping rope, I would recommend buying a how-to book or searching the net for technique tips. While this would most likely be time-efficient for your learning curve, you can also just jump right in, so to speak.