There are tons of fun ways to celebrate a birthday! Feel free to read my trip report from January 07, linked in my signature. My DD turned 5 on that trip, and she had a truly magical birthday.
When you get to WDW, you can request a Birthday pin for the Birthday girl to wear. You can get this at your resort or at guest services in any of the parks. People will see her pin and wish her a happy birthday for the whole trip!
I spoke the front desk at our resort and arranged to have balloons and a card left in our room from DD's favorite character (Sleeping Beauty.)
Schedule a fun character meal on her birthday. (Or if you're like us, schedule one for each day of the trip!!) Make sure to mention that you are celebrating a birthday on your reservation - most sit down restaurants gave my dd a special bday cupcake after every meal for the entire week!

Some places also had special cards or a laminated placemat for her as well. 1900 Park Fare even gave her a bday tiara. You can also special order a birthday cake if you like. We did this at 1900 Park Fare for her bday dinner. Everyone in the restaurant sang to her!
If you do the Bibbidi Bobbidi Boutique on her bday, she will receive a special Birthday sash that is different from the regular BBB sash.
There are so many fun things that you can do. I hope your DD has a magical birthday!