What about giving people who buy an annual pass a ticket booklet of sorts? (Yes, I know this is like Six Flags, but you know just because it's Six Flags doesn't mean it's not a good idea.)
Anyway, back to what I was saying. When a person gets a new ap (or renews) they would be given a booklet with one free admission to any one of the other Disney parks. Then after that the ap holder is given 35% off park admission for the rest of the validity of the annual pass.