You have time to cover different types of activities and interests.
You could start with a selfguided walking tour covering the historic sites and museums which are hidden behind the glitzy tourist waterfront and shopping mall part. Maybe 2-3 hours.
This might be followed by a tour around the Island, either by taking one of the many van tours offered at the port or on your own. 2-3 hours?
You could leave resp finish the tour in the beach and hotel area for what kind of relaxtion you like - Sun, food, Drinks, water or Land "fun" activities.
Back to the town maybe a short look for souveniers before going on your sunset catameran cruise.
Just ideas.