Make sure you go to City Hall at MK to get a "Today is my Birthday Button" They also have birthday buttons at AK, MGM and Epcot - just ask at Guest Relations.
Make sure you mention the bday at the sit down restaurants you dine at - they usually will come around and sing at your table and bring a cupcake for the child. Sometimes you can arrange for a cake (at a cost) Also mention it when you make any PS's.
I found that most things that happened were impromptu as a result of DS wearing his birthday badge.
We spent the day at MK, and at MTT a cast member asked me if it was alright to give DS a free bag of mickey gummies since it was his birthday. Also when we went to see Tweedle Dee and Tweedle Dum, the CM noticed DS's button and brought him to the front of the line to have everyone sing HB to him (with Dee and Dum leading the choir!)
We had dinner at 1900 Park Fare, they brought a cupcake and he got to keep the placemat with "My special day" on it with drawn pictures/autographs of all the characters that appear at the meal.
Quite a few characters autographed his book with HB.
And finally he got numerous HB's from various CM's and guests on his special day!
You can also arrange just about anything - if you are willing to pay for it! For example, we had a MK fireworks cruise booked at $120. They offered to make it a birthday cruise (cake and decorations on the boat) for another $100. (total $220.!) I declined!
We also had
www.giftsofalifetime.com deliver a birthday basket to our hotel on the morning of his birthday. You can have them "tailor make" them according to what you want and the amount you want to spend.
I hope this helps.