I don't believe that any of the tour operators offer a stop to the ruins on Ambergris Caye, but you could ask around when you get here.
The Altun Ha ruins trip is offered by several Caye Caulker tour operators, but unless you go on a non-cruise ship day, it could be overrun by tourists. An easy day trip is Lamanai. You can go on your own, or you can arrange a tour, depending on how adverturesome you are.
If you go on your own, take an early water (6:30 a.m.) taxi to Belize City, then cab or bus North up to Orangewalk, where you get on a boat (where the river crosses the road-you'll see it, there are signs) and enjoy the river ride, with orchids and bromelaids dripping off the trees. Just dont forget, the last boat back to Caye Caulker, from Belize City leaves at 5:30 p.m.
[This message has been edited by Cybercayecaulker.com (edited 11-17-2003).]