My favorite place in San Pedro is San Pedro. Just wonder about aimlessly and you'll have a good time. It is difficult to find a bad meal in San Pedro. A snorkel trip to Shark/ray alley is a fun, reasonably priced tour. See if you can hook up with Alfonso. Ambergris has one certified ruin called Marco Gonzales, I believe. It is at the extreme south end of Ambergris, but you would not notice it if you didn't know what you were looking at. My best advice would be not to make up an itinery of things to do...just let it happen.....Belize Style.