It's true that the sheeple with ship's tours get tendered in first, but with all the water taxis to and from Caulker, plus the option of flying one way (if you fly to municipal and if Maya Island still has the 25% off deal for cash, the cost is only around US$22 one way) you shouldn't have a problem getting to the island and back.
The zoo is a great idea.
In the city the Museum of Belize and the House of Culture (Government House) and St. John's Cathedral are well worth seeing.
If you decide to stay in Belize City for lunch, Harbour View is a pleasant place with views of the water, but the restaurant at Bird's Isle is more Belizean, or you could taxi to the Riverside Tavern for the best hamburger in Belize. Or you have all the old places to choose from like Nerie's and Macy's. Also Big's Daddy's.
--Lan Sluder