We went on a private tour organized by a guide. The boat trip was just the right length--be warned that you need to wade a bit though!
We did snorkel but found that the water had stinging jellyfish and the coral was not very exciting. We went to visit the tortoises and it was clear that our guide was very knowledgeable. You could go right up to the tortoises and stroke their heads. We saw tiny babies and enormous tortoises. the rest of the island is pleasant and we walked about. we spent a little while on the beach and had a pleasant time swimming.
As a half day trip this was just right.