Make the most of this day of active rest. You're going to want to get in your 30 minutes of cardio, but there's no reason to constrain yourself to a brick-and-mortar gym.
Try to switch things up and trade in the treadmill, elliptical, or stationary bike for a jog outside or a few laps in the pool. Remember that doing cardio on your active-rest day is meant to keep you agile while still challenging the body. A hilly bike ride might be the perfect go-to—take in the fresh air and atmosphere, so cardio doesn't seem like such a tiring task.