It's not necessarily the cheapest, but what you want to do.
I tell everyone to rent the car which gives you more freedom.
Pros - You can go anywhere when you want. Visit I-Drive, restaurants, other parks, visit Publix to stock a cooler, and you can explore.
Cons - You pay to park at the RPR $9 each night. Gas is expensive. Rental car fee can add up if staying for a week or so.
Most recommend limo services. Mears is the big one at Disney, they have phones in the lobby. There are a few more that advertise on here that others may recommend.