this is one of the most often asked questions on this board. Many have done it and many love the idea of renting a car. I, personally have done it at least a dozen times. There are many car rental places within a mile or so from the cruise terminal. Payless, Budget,
Avis, Alamo, Enterprise, Thrifty, Hertz and I am probably forgetting one or more. I know there is a Diser who has a pic map of where they are all located. I recently used Alamo (which is also the location for Enterprise and National). They are literally 5 minutes from the port.
We usually rent at MCO and drop off at the port before the cruise. If it is a short cruise I sometimes keep the car parked at the port and will return it back to MCO. Some rental places charge one way fees, some don't. I always go to
www.mousesavers.com and check out their "transportation" page that lists all the rental agencies at WDW, MCO and the port and even has the latest discount codes. It can be time consuming to check each code but many times it is worth it. We also recheck our reservation often as prices change all the time. So far rental companies do not charge anything to cancel and rebook. If you find a better price after you booked a car make sure you book the new reservation with the better price and then cancel your other reservation.
They all have shuttles to bring you back to the port after you drop off the car but many choose to drop luggage and family off at the port and then go return the rental. Reason being it is often easier to fit one person on the rental than dealing with everyone in your party and all their luggage. By dropping luggage off at the port first you don't have to deal with unloading luggage from the car and then loading it on the shuttle. Just food for thought.
MJ