My husband and I just returned from a week in Ambergris Caye. We loved the laid back beach attitude, Did some snorkeling half day trips.
I suggest: Hol Chan Reserve.Shark Alley snorkle trip
AND
You must do a trip to the ruins. We just did the day trip and it was so awesome. The bus ride seeing the mainland. Brought us back to our days in Africa, and gives you a good perspective on life.