There's an entire website about the "Secret Restroom" located to the side of Carnation Plaza at Disneyland. It's not so much a secret anymore, but it is a fully-accessible one-holer tucked away as to not be noticeable. It's also cleaned every half-hour. From the FAQs:
Why is the Secret Restroom better than other Disneyland restrooms? For a variety of reasons. For example, it is the only restroom in the entire park that is entirely handicapped accessible, full of cutting-edge technology, and against park policy for discussion by any cast member.
When was the location of the Secret Restroom first publicly revealed? Although Disneyland goes out of its way to avoid publicizing the Hidden Restroom, a photograph of it (seen at right) was printed in a "Disneyland Secrets" article in the Orange County Register in 1986.
The site has full pages dedicated to every amenity and fixture of the restroom. Link -via Boing Boing