Take a day trip to the Dry Tortugas National Park.  There is more information about the park under things to do in Florida in general, or from the National Park Site.  You can go by boat or seaplane. The boat works in conjunction with the National Park Service.  Boat leaves at 7:30 am, provides breakfast, lunch, a guided tour of the fort, and free snorkeling gear.  Returns to Key West about 5:00.  A fun and informative day.  It's really hot out on the island.  Come prepared with sunscreen, swimsuits, and towels.

If you want to go snorkeling at the reef then there are a few boats that will take you out. There are several big catamarans that go out a couple times a day. If a more private experience is what you are looking for you can try Floridays which is a 60 foot sailing yacht behind the Hyatt hotel that does reef trips and sunset sails every day as well as other smaller boats that offer a more personal experience such as Fun In The Sun Charters or Lost Reef Adventures. Scuba diving is also a must do in Key West. There are great dive companies in Key West including Dive Key West and Lost Reef Adventures that can offer reef and wreck dives daily. You can also do a resort course and dive in one day!