There are three main day trip islands on the Great Barrier Reef - Green Island, Fitzroy Island and the Frankland Islands. Some have resorts on them, some don't.
Green Island is a coral cay.
This means no mountains or land forms, just palm trees and beaches. There is quite a lot of coral around the island and is great for those people who have never tried snorkelling before and are a little nervous. Green Island provides a good solid base to work from and the water is quite shallow. Water sports are available for hire from the island as well, and there is a resort on the island with a swimming pool that is available to day trippers at a small expense. Scuba diving is also an option on Green Island.