Not really sure what you are looking for. I have been to St John and Belize a number of time. Both are English speaking places. St John has much better beaches and snorkeling from the beach. You need to take a boat to get to the reef at Belize but the snorkeling (or diving) is much better. Both are good places to chill out. Caye Caulker is much more laid back than Ambergris Caye or St John, but it is a very small island.
Belize offers excursions to Mayan ruins, and snorkeling with rays and sharks.