If the weather's nice, head outside to run laps on a track or play soccer with some friends. If it's nasty and cold, swim laps in an indoor pool, or just brave the elements and indulge in a fun winter activity such as skiing or snowboarding.
Whatever you do, it should get your heart rate up for two hours. Just because I want you to enjoy yourself doesn't mean you can slack off and take it easy. If you're not sure whether your favorite activity is enough of a workout, invest in a device that measures your heart rate while you're on the go.
Take time to do some ab work today as well. Bodyweight moves like crunches and air bikes can be done practically anywhere, and require no special equipment. So no matter where you do your cardio, just drop to the ground and hit your abs!