You can rent a car from Avis (at least that's what we are planning) at the Orlando airport and return it to AVIS at Port Canaveral. They have a shuttle bus to and from the ship. So, you rent a car "one way" on each leg of the trip. In your case you would want it maybe overnight the first night, then return it in the morning to the rental company and get shuttled to the ship. Then do the reverse when you are going back to the airport.
I found the cost to be very reasonable and less than the cruise ship's transportation costs. BUT, if your hotel has shuttle service to the port, you may want to try Cruise Fanatic's link and see what they would charge to get you to your hotel. Then, maybe you only need to rent the car from the port back to the airport?
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