Exploring a new fitness hobby together is a great way to have fun while getting in shape. If you like being outdoors, go hiking. Or, get a gym membership so you can motivate each other to work out. Choose a time of day to exercise that's convenient for both of your schedules; it's so much easier to get to the gym consistently when your other half goes with you. Couples who are competitive by nature can register for a 10k road race and push each other to run farther and faster. Not everyone can afford a personal trainer, but if it's in your budget, hire one and train together. Whatever you do to shape up, reward each other when you achieve a specific fitness goal -- show your sweetie how proud you are!
Signing up with other couples and friends for a co-ed sports team will whip you into shape, but you'll barely notice while you're having so much fun. BYOB kickball is a definitely a blast, but maybe shoot for another sport that will burn a few more calories. Sports like softball, basketball and soccer will really get your heart rate going.