When is the 15 hours? Overnight or after a morning arrival? If you're not trying to go during rush hour, it's less than 30 minutes to the city by Uber for a cost of ~US$12. If you don't have baggage to carry around, you might be able to go do the canal tour or go to a mall (e.g., Multiplaza) and still have plenty of time to spare. I like Panama, but the (current) airport is very boring. The new airport being built next door presumably will have more to do.