Tandem cycling just might be the best way to exercise with a partner. Unlike other activities (like running) where twosomes of different levels can slow one person down, doubling up on a bike is a breeze. The stronger rider sits up front and does all the shifting, steering, braking, and heavy pedaling; the weaker cyclist rides in back and kicks in extra power. Take the effort level to a moderate intensity and you'll both burn about 500 calories an hour. We guarantee you'll pick up the rhythm instantly-even if the last bike you rode had a banana seat.