When an activity is fun, it's much easier to stick with. That's the underlying philosophy at Lime Fitness, whose name doesn't refer to a color, but to an acronym: "Lifestyle Improvement Made Easy." Here, enthusiastic instructors believe that fun and fitness can be synonymous, encouraging students of all stripes to enjoy themselves while breaking a sweat and getting in shape. Teachers up the engagement levels by roping in everything from dancing to core workouts during classes such as Zumba, Bellyfit, and Piloxing.