Most cruise ships have the packages available and they can be arranged before they even get into port. That is probably the best choice because the cruise lines know how much time the ship is in port and they have excursion packages with guides who regularly do this specifically for ship passengers.
I do think they will be able to see ruins, maybe a zipline or cave trip, even a snorkel or dive trip. Surprisingly, they can do a few things. Whatever they do, have them go outside the city to experience the real beauty of the country.
When I mention my love of Belize and desire to move there to people whom I know have worked on cruise ships, they all wonder why and comment that Belize City is a S(#(hole of a place.
Unless, of course, you want to keep the beauty of the country a secret....then tell them to just tour the city.