Be sure to get her a birthday button for free at guest services on your first park day--she can wear it for the whole trip, and everyone will be telling her Happy Birthday for the whole trip. She might get a surprise or two because of the button, but you can't count on that, so I wouldn't mention it to her.
At her age, she can definitely dress up like a princess at the parks if she wants to, although if you plan on doing that, I'd go with costumes that are comfortable (not itchy).
Those are just a few things off the top of my head--I'm sure I'll think of more later!