I can look up some information regarding the various car rental companies that service the cruiseship terminal at Port Canaveral, as well as giving you a resource for options other than rental cars.
I am sorry that I cannot recommend the "best rental situation"... that will probably depend on what exactly you're looking for the rental (pricing, vehicle type, etc) and will be specific to the company, dates and times that you choose. I recommend that you compare various car rental companies to make sure that you're getting the best rental situation for you.
Port Canveral's website (portcanaveral.com)
lists the following companies that service the cruiseship terminal:
*Note that in this situation - car rental to drop off at Port Canaveral cruiseship terminal - these companies are offsite (2-5 miles away) and you would basically have to drop off the rental car that you pick up at the Orlando Airport and take the car rental company's shuttle service to the cruiseship terminal. When you return, you would call the car rental company to request shuttle service back to the location to pick up the return vehicle. You would have two seperate rental contracts.. one from the airport location and one from the cape canaveral location .
(car rental company links open in new windows)
Avis - 6650 North Atlantic AveAvis.com local 321-783-3643/toll free res line 1-800-331-1212
You could also look at the possibility of using a shuttle service, either to avoid renting a car at all, or if you have a rental car and want to return the car to the Orlando airport, instead of the cruiship terminal. According the the Orlando Airport site regarding Cruise Transportation:
Some cruise lines offer transportation on the 42 mile route between Port Canaveral and Orlando International Airport, therefore you may wish to check directly with your cruise line.
Alternatively, check the Out Of Town Shuttles page of their web site. or their listing of Vehicles for Hire" which lists companies which are permitted to pick up passengers that have pre-arranged transportation with the company.