i went a few years ago, and one of the best things I did was a day trip to one of the many islands off the coast. i purchased the trip through the hotel, and it was probably ~20 people total. The island had a small hotel (didnt stay there) and the trip included a lunch of fresh fried fish which was delicious. I dont remember the exact island we went to, but I'm pretty sure there are alot of these trips of similar quality.
I think we were on the island for 5 or 6 hours, and went out with a guide to go snorkeling. the water was crystal clear and saw quite a bit of fish. You can also just lay on the beach and enjoy the sun.