Another option is if you're staying at one of the hotels bordering Downtown Disney. Just walk from your hotel to the Downtown Disney bus stops, and use Disney transportation all day. There are a few hotels there less than a 5 minute walk from the Disney bus stops.
The only catch here is that none of the Downtown Disney buses go to the theme parks. You'd have to catch a bus from there to one of the resorts close to the park you want to visit, then catch a different bus, the monorail, or a boat to your preferred park.
David