You can get a free Happy Birthday button (not a pin) and plenty of castmembers will wish you a Happy Birthday, but that's the only things that are guaranteed. I think a couple restaurants will sing HB to everyone who is there at the time.
Anything else will depend upon the castmembers. You might get a free cupcake or birthday dessert, a free beverage, confetti, a card, HB sung to you, a signed photo or you might not get any recognition at all. It is best not to expect anything and be pleasantly surprised if your day is recognized because probably half the people visiting DW are celebrating something - birthday, anniversary, graduation, engagement, honeymoon, first visit, family reunion, a life changing milestone, etc. so it is not possible to recognize everyone individually.