Agreed on the hotel points. Probably the best selection of places most like a high-end hotel chain is toward the north end of Ambergris Caye, but there are a few nice ones close to San Pedro as well. I second VRBO and TripAdvisor -- I've used them for both Ambergris Caye and Caye Caulker.
Air R, thanks -- that's a handy site. One caveat when it comes to Belize, though: it claims you can fly DCA/IAD-BZE 1way for 14.5k Avios, and technically you can via CLT, but as far as I can tell the CLT-BZE-CLT non-stops only exist on weekends. You have much better availability via MIA for 15k.