Where: Underwater. Ask German, Cayo Espanto's very own dive instructor, to take you on a private guided dive at Ambergris Caye's barrier reef. Pen "Will you marry me?" on a whiteboard before going underwater.
Why: It's adventurous, it's original, and it offers the ultimate in privacy. (OK, so German the dive guide will be there too, but you can ask him to hover behind a rock.)
Where to stay: Check into a beach house on the private island of Cayo Espanto, three miles off the coast of Belize—each of the six villas comes with their own "houseman." Fancy.