If you are eating at a sit-down restaurant and you have told them it is for a birthday, there is a chance (not a guarantee and I don't even want to say likelihood) that the birthday person might receive a free birthday dessert. If you are at a buffet like Goofy's Kitchen, the server will probably bring a cupcake and sing happy birthday, maybe with a character or two in attendance. Other than the button and the well-wishes, that's about it as far as free goes.
If you're looking for stuff to buy, there are "happy birthday" Mickey ears and t-shirts and maybe a pin. There are probably some other birthday themed things in the stores that I haven't paid attention to.
If you're just wanting to figure out what kinds of "special" things to do on the actual birthday to make the day feel more exciting, search the board here for threads on special activities. Things like riding the Lilly Belle, ringing the bell in the wheelhouse of the Mark Twain, or booking a dining experience are considered "special." Of course, if this person has never been to
Disneyland, then pretty much everything will probably be new and exciting, and some of the things I just mentioned would probably seem more special to a frequent parkgoer or someone really steeped in Disney lore.
