You could do it on your own. We just came back from Port Canavreal and we rented a mini van, one way for about $60. The rental car shuttles pick you up from the ship and take you the rental agency. Then you could drive yourself to NASA and then to the airport (The airport is only about 40 minutes away.)
Then you are not at the mercy of the excursion schedules.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013