What would really knock the tiara off your birthday princess? Surprise her with a princess doll or Barbie cake.
It may sound a bit intimidating to try this one at home but essentially what you're making is the skirt -- the more elaborate like a fluffy ball gown the better. To construct this, you'll need two round cakes and one dome cake (or purchase a specialty baking pan in the shape of a doll dress), frosting and a doll (or specialty cake doll), and whatever candy or other goodies you want to use for skirt decoration.
Not only does she get her cake, but she gets to eat it with her new doll, too.