You're only there until 2pm and the beach is a half hour cab ride away. We went to Lucaya Beach, which the Shore Excursion desk recommended, but it was very small and somewhat dirty with no chairs, umbrellas or facilities. It was also a long walk down a gravel path through the police station, and next to an abandoned resort hotel. Not worth the time to get there and back in my opinion. However if you do find yourself at Billy Joe's cafe on the beach, make sure to try the grilled conch.
There's nothing to do where the ship docks except maybe a gift shop. I would recommend just staying on the ship.