High knees are an amazing workout for muscle tone and muscle endurance in your legs, and they’re easy to do just about anywhere with no equipment at all. Simply alternate between bringing one knee up as close to your chest as possible, then the other--like running in place but emphasizing the upward motion. The trick here is to get your knees as high as you can, as fast as you can. Aim for 1 minute non-stop with a minute rest in between sets.