We've rented a mid size car (5 of us) on-line from National. They have a site right at the Airport. Walk out from baggage, up one floor in the parking garage, walk down the aisle pick your car and drive to the drive-up booth, it was so easy. Arrive Friday, stayed at all star for two nights, checked in at Broadwalk on Sunday then returned the car to the dolphin valet parking (i never had to go into a building). Cost was $46.00
This worked out great we were able to go to Sea World on Saturday, grocery shop off site and eat at our favorite offsite restuarant (Logans).
We've done this twice, once we just hired a limo for the return trip to the airport ($45.00) and once we picked up another car at the dolphin the day before we were to leave.
We've always have had a car before this so it was a little scary the first time not having a car readly accessable to us, but with staying onsite we found we didn't need one the whole time we were there.
Good Luck