Belize City was very dirty, and we were happy to do our tour booked through Dorian (www.privatebelizeadventure.com) to Xunantunich. If we hadn't done that tour, we wouldn't have felt comfortable leaving the terminal area. We heard that the cave tours were nice as well.
Less than happy here. The beach was not overly nice. We were happier going back to the ship. Maybe if we took an excursion, but I'm not sure that would have helped.
I think that Carnival missed the boat spending so much money - they need to invest more in the beach.