My BF has taken me the last few years for my bday and it has always been wonderful! You need to get a bday button from guest services (and I say wear it for your entire trip!). At all your restaurants tell them a bday is being celebrated. You may get a happy birthday or you may get more! I have had my table decorated,cupcakes with candles,cards signed by characters,singing,a crown, being the shakka maraca leader and lots more!
My parents weren't able to go so they had a cake delivered to our room! It was delicious!
I also recieved a card in my room from Mickey,my fav! By BF told the front desk it was my bday!
Word of caution: Go expecting you will have a great time because you are in Disney. If any of these extras happen....then that is just it a wonderful extra surprise! If you want something specific you will have to create it on your own or ask!
Have a great time!