You've likely heard the term 'spinning' before, either from wealthy Marin housewives or the spry yuppies in your neighborhood. But if you don't know what it actually entails, spinning is simply an aerobic activity on a stationary bicycle called (drum roll) a spinning bike. Developed by cyclist Jonny Goldberg as a way to train for bike races, it's great because it also includes the instructor walking participants through a visualization exercise that makes you feel like you're in the big outdoors. Throughout the class, pace varies accordingly with the hills and dales you're asked to imagine. The focus is very meditative, and works wonders on both your gams and your stress. It also burns about 450 calories in 45 minutes and tones outer thigh muscles to boot. Additionally, it's simpler than other aerobic exercises that require a certain amount of coordination and you can do it, regardless of what your fitness level looks like.