Hi, Lisa! I happen to personally know that September 16th is by far the most wonderful day to celebrate a birthday, and I hope your little princess enjoys a fabulous time celebrating at Walt Disney World!
If you have already booked your vacation, call back and make sure you let them know that you are celebrating your little one's birthday. You just never know what pixie dust might land on you if they know in advance that you have a celebration planned.
Don't forget to get a birthday button. The buttons are free at guest services, and they will personalize it with your daughter's name. We've always gotten a bunch of happy birthday wishes for our kids whenever they are wearing their button.
If you are making advanced dining reservations, again note on every reservation that you have a birthday girl in your party. There are sometimes some table decorations or special desserts, but this varies by restaurant.
If you want to do something on your own, I suggest contacting the Walt Disney World florist. You can see some of their amazing birthday packages by visiting their website at
http://www.disneyflorist.com/ . You will find everything from an elaborate birthday celebration to some great gifts for a smaller budget.
Have a magical vacation!