Our longest trip was 10 days so far, but I am always dreaming of ways to go to WDW, and stay there longer when I get there. If you are flying in, I would suggest renting a car at the airport , then keeping it a few days to go to the grocery store and do any off-site touring. You could then return the car at the car care center at WDW, or at one of the Downtown Disney hotels, depending on the company you rent from. The best thing to do would be to price the car rental, deciding exactly how many days gets you a good price(it is sometimes less expensive to keep a car a week than for just 4 days), then plan your itinerary accordingly. Once you return the car, you can rely solely on Disney transportation to their parks. Another alternative would be to use a towncar service from the airport(in order to get your grocery stop), and again to make a day-trip to Universal or Seaworld.