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. In my opinion, the fish and coral around Green Island is a lot more plentiful than off of the beach at Fitzroy Island. There is less people at Fitzroy Island as it is not very well known for it's coral and fish life. There is also a resort on this island, but the pool is free to use for all day trippers.