Good old fashioned skipping, still brilliantexercise now and great fun to do with clients. For the best results compete with your clients, either who can do the most or who can do the most in a set time i.e. 1 minute, garenteed to get them to maximise their effort. Boxers have been skipping for centuries and they are extremely fit. Variation is the key, there are standard two foot jumps, where you do mini jumps with both feet off at the same time, alternate feet, where you kind of jog on the spot whilst skipping. Then high knee skips either both feet together or alternate legs, aiming to get your knees up towards your chest with each jump and many more. The secret is good rhythm with the rope, so consistent speed makes the whole thing easier. Good luck and enjoy. Let me know if you have any problems or questions.