We are flying into Orlando, doing a 4 day Disney cruise, then going to Universal for four days.
I found it the most cost effective to rent a mini-van for our family of 6. To rent a van for 8 days, it's costing $300 (Dollar). Since we are flying in a day before the cruise and staying at the Marriott in Cape Canaveral, they have free parking, so I can leave the rental there during our cruise.
We will take a shuttle from the hotel to the port. It's actually cheaper to keep the van for 8 days than to rent for a day, return it, then pick it up again after the cruise. However, if you don't have a hotel to keep your car at, that may be the only option for you and there wasn't a huge price difference.
This will also allow us to have transportation while at Universal (although you do have to pay to keep the van in their hotel parking lot) and also give us transportation back to the airport.
There are lots of rental codes out there for the different car agencies. I have several reservations right now, but found Thrifty (off-site at the airport - $250 for 8 days) and Dollar (on-site) to be the cheapest so far.
Good luck
