Doesn't some high- speed jump-roping sound like F-U-N? Well, the upgraded playground pastime, burning more than 1,200 calories per hour and averaging 135 revolutions per minute, has grown into one of the most superior cardio workouts-one that former Middleweight Boxing Champion Michael Olajide Jr. has made into an instructional video and audiocassette called Aerojump. The 60-minute full-body workout is said to be easy on the joints, and it simultaneously enhances agility and endurance. With performance-enhancing drills, tips to intensify the workout, and programs designed for in-home use or travel, the tape and Olajide's specially designed poly-nylon jump rope offer an exceptional alternative to the latest infomercialed machine. Video, $20; cassette, $12; Rainmaker Jump Rope, $15. Seek out http://www.goaero.com for the 411.