From Ambergris and Caulker, and from smaller cayes with advance planning, you can do day trips to the mainland to see Mayan ruins, try cave tubing, visit the wonderful Belize Zoo, and do other activities. However, the cost will be significantly higher than if you did the tour from a closer point.
Tour operators on the Cayes run trips to Mayan sites such as Lamanai (usually a full-day trip by boat and road) and Altun Ha (normally a half-day trip, although it may be longer if it includes lunch and a spa visit at Maruba Spa). If you're up for a trek, you can visit Tikal on an overnight trip by air to Flores, Guatemala, with a change of planes in Belize City. It's also possible to see the small, unexcavated ruins on Ambergris Caye—including Marco Gonzalez on the south end of the island, reachable by golf cart or taxi, and Chac Balam at Bacalar, reachable by boat.
Scuba Diving and Snorkeling
Diving near the cayes is world-class and snorkeling is ubiquitous. Most dive-trip operators (who are usually also snorkel-boat operators) are on Ambergris Caye. Many operators also offer PADI courses. San Pedro has Belize's only hyperbaric chamber and an on-site doctor. Many dive shops are attached to hotels, where the quality of dive masters, equipment, and facilities can vary considerably. It's a short boat ride to the spur-and-groove formations along the Barrier Reef and to Hol Chan Marine Reserve. Several San Pedro operators with speedboats can take you to the Blue Hole, the largest ocean sinkhole in the world.