As others have said, you will get a Birthday Button and many many Cast Members will wish you a happy birthday. It feels kind of nice to be recognized in the park. Also, I noticed on this last trip that many other Park Guests wished me a happy birthday. Were they off duty Cast Members? Who knows. But it definitely felt nice, and I came to the realization that I should be saying happy birthday to other guests too! It was nice to have a little more friendly interactions with other guests.
Aside from that, nothing is guaranteed. When I was walking around the Canadian Pavilion in EPCOT, I stopped to buy a popcorn. The Cast Member said, "Don't worry about it, it's on Mickey today".
I've gotten a silly straw at Cosmic Ray's and the Cast Member asked if I wanted all the other Cast Members to sing happy birthday.
Tusker House has given me a card that has the characters signature, and a cupcake. The characters on the card were the same ones that were meeting for breakfast.
Depending on your server, Whispering Canyon will usually make you do something embarrassing. I was told to get my cupcake I would have to get up and sing Let it Go to the whole restaurant. After I did, my server was about to hand me a cupcake and another server stepped right in front of me, grabbed it out of my servers hand, and at the whole thing in one bite. It was hilarious.
All of this stuff typically happens when you aren't seeking it out though. Going up to someone and saying, "Hey, do I get anything because it's my birthday" will more than likely not work in your favor. Not saying you were planning on doing that, just a bit of advice.