Try and find out about a snorkeling trip. We went on a half day trip to the other side of the mainland, but it was well worth it. We went out by boat to 2 different reefs, and even got up close and personal with a shark feeding!
Paradise island has an interesting lighthouse on its grounds. it can be best seen either coming to Nassau by boat or else from Fort Charlotte. it is supposed to be the oldest lighthouse in the Bahamas and - maybe - even the oldest in the West Indies.
The lighthouse is located on the western rocky tip of the island and it is still being used... a white light indicates that you can enter the port safely, whereas a red light indicated danger.