Instead of paying big bucks for access to a beach club, rent a Jeep and explore the island (it's only 30 mi/45 km) for the full trip.
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Go on the 3-stop snorkel trip with Captain Marvin, which includes Sting Ray City - a unique experience.
Go to the southern most point of Florida (only 90 miles from Cuba). Not much there, but it's a famous landmark.
Don't miss the opportunity to enjoy a leisurely breakfast/brunch in the Blush Dining Room at the back of the ship. This is a seated/served meal in an elegant setting (at no additional cost).