Ambergris Caye. There's a "fast" 2-hour water taxi from downtown Belize City but the best way to get there is via Maya island Air or Tropic Air. If there are a bunch of you take a taxi from the International airport to the Municipal airport (About $50) and enjoy a significant discount (about $50 per person negotiable).
On Ambergris: Coral Bay Villages would be my 1st choice.