I'm with PP... rent a car before your 5 day, park it at the port. Get off the Magic and drive away. Park at Port Canaveral, too, so you don't waste time with a rental return. My reasoning is that it's only a 3 day cruise. The Dream is bigger than the Magic and you'll want as much time as you can get on it to explore and enjoy the ship. Since you'll be arriving, at best in the afternoon, so others will have a few hours to have gotten settled before you even arrive. The parking and the rental will be more costly (the rental could be fine if you watch for deals, as you'll have it for at least a week), but given what you are trying to do, I think it will take a lot of stress off an already stressful day.
Are you going to still do 3 days in WDW or at least one before returning home?