You may want to look at a couple of the airport area hotels. Not sure if they still have a package, but I think the Crowne Plaza and the Wyndham have a parking package available, in addition to have shuttles to the Port of Miami.
Also, you may want to look into the option of a one way car rental each direction. Hertz has a shuttle from one of their downtown locations to the Port, while Alamo and Thrifty have free shuttle service from their airport location to the Port.
Carnival Triumph 11/2005
Carnival Glory 9/2006
Carnival Freedom 11/24/07 - Open Bar Cruise!!!