If there a ride she is particularly excited about? Tell the cast member at that ride (or all the rides) that it's her birthday and ask if she can sit at the front. They usually are more than willing to assist you to help that request come true.
Plus, cast members are great at wishing guests happy birthday if they see the button - so be sure to pick one up!
No promises, but sometimes at restaurants/quick dining areas, cast members will sing out a special birthday song for the celebrating 'princess'. It's really nice and will make her feel like she is the center of attention!