Food, food, and more food!
Have at least one nice dinner at a nice restaurant, whatever your favorite one is. If it's one that has lunch offered, you could even do that - it will still be just as special, taste just as delicious, but be a lot cheaper!
Like they said, definitely get a birthday button. Some of us custodies at MK would get birthday people free ice cream or popcorn - Some of the QSFB outdoor carts people will give them to you as well! And like they also said, sometimes Attractions hosts/hostesses will take you to the front of the lines, etc. It's always beneficial to get a button. And get the rest of the people in your group to wear "I'm Celebrating" ones as well!
If you like attention, just make it obvious that it's your birthday. When the parade attendants are out amusing the crowd before 3 o'clock, let them know! There's no telling what they may do for you.