Well, we surprised our son when we went for my birthday a couple of years ago (he was 4). We told him we were going to the Bronx Zoo but had to ride a bus (at the airport). It worked.
As far as things to do for his birthday, we go on his birthday every year and just got back for my other son's 3rd birthday. Here are some ideas:
-If you're staying on the property, make sure you tell the people at the hotel which day is his birthday. They bring balloons and a signed picture from his favorite character.
-Make sure you get your birthday button from customer service at the park you go to. Everyone says Happy Birthday and many times CMs will do nice things for your birthday child.
-Make sure you tell the people where you eat. Of course, they sing but some places do cooler things than others.
-We'll usually pick up a little toy for each day we're there and give it to him each morning for his, "birthday week." There's PLENTY to buy there for sure.

-They have birthday hats and even shirts now you can buy.
-It's a little late but you may be able to get into a character breakfast during that week. Let them know it's his birthday.
Birthdays are great in DW!