All the cruise ships come into Belize City. The harbor is too shallow for the big ships to dock, so passengers are tendered in. The Fort Point Tourist Village is where you'll disembark -- there are around 30 little shops there. You can walk to other points of interest such as the Museum of Belize, House of Culture, St. John's church and others.
I think most people on this forum would say that Belize is well worth seeing!
The most popular cruise ship tours are the snorkel trips to Shark-Ray Alley and elsewhere, and the cave tubing trips. Trips to Lamanai, Altun Ha and Xunantunich Maya sites are also popular.