Welcome aboard, Margaret! I bet you've begun your countdown, as it's not too long now until you'll be setting sail on the Disney Fantasy. When you arrive, Cast Members will announce your family name and applaud your arrival as you walk into the grand Atrium lobby. What a magical way to begin your vacation!
Most of the major rental car companies have locations in Cocoa Beach or Cape Canaveral which is just a few minutes away from
Port Canaveral. Most of them will offer free shuttle service to the port. I'd suggest choosing the rental company that you prefer at the airport then verifying that they have a location in the port area and that they offer the shuttles. I believe that Budget is the closest, meaning less time in the shuttle. Avis, Hertz, Thrifty and Enterprise are not far either.
You might then consider booking Disney Cruise Line's ground transportation after your cruise if you're heading right back to the airport. There is an additional cost of $35 per person each way, but letting Disney take care of the details, especially as you disembark after a relaxing and fun week, is the way to go in my opinion!
Happy planning, Margaret! Please let me know if I can help with anything else before you set sail!