When you get to Belize City you will be tendered into the shore. The area has a fence around shops in a two block radius. Tee shirts and over priced jewelry that you see everywhere else.
We attempted to go out of the fenced area and were swamped by guides that wanted us to rent them for the day. Very aggressive. We walked to the left seeking out the city center to see some culture and buildings. We have been to many islands in the Caribbean but when we crossed the bridge into the city we didn't make it one block.
We returned to the "safe" area and went to the one restaurant that you could get a drink and lunch. Drinks were terrible and after an hour and fifteen minutes lunch showed up wrong, cold and tasteless. We paid a bill and headed back to the ship. I guess one one hand we saw it. The boat ride back out to the ship is pretty long as well.
You may need to do some research on an excursion away from the city but I would check it out. We were pretty disappointed because we have always seen great portraits of Belize on the travel shows and hear about all the expats retiring there but rest assure it is not in Belize City or close enough for a day trip.