traveling out to any of the caye islands is wonderful for snorkeling. i spent a week at long caye using slickrock adventures which was paradise, but i've heard good things about ambergris and tobacco caye as well. also had a great diving experience at the blue hole.
placencia down south on the coast was a lovely setting. i found some guest house on the beach (kitty's place, if i recall) which was nice.
and of course, mayan ruins abound. close in to belize city is a place called lamanai, which is an easy half day visit. more interesting ruins lie deeper into the interior of the country.
in belize city i stayed at the glenthorn manor guest house, which was a bargain and incredibly comfortable. i highly recommend it (though i had the whole top floor to myself, so perhaps that made a difference). i know we're all about points on this forum, but please don't wither away in the belize radisson.
i believe marva's was the name of the joint in the capital where i found the belizean food to be pretty good.