If you can manage it, go over to the DISigns thread and get a custom Birthday graphic done up, and iron it onto a plain shirt for her to wear for the day. I have also seen door knob hangers indicating a birthday celebration.
I am taking DD to DLR for her 8th birthday, and making a special autograph book just for that day. If you don't want to go that route, get some birthday stickers and cover a store bought one so that it becomes a record of birthday wishes for her.
If you are doing character meals, try to have her favourite one that day, and make sure they know it is her birthday.
I would do the suggestions already listed for decorating the room.
I hope it's wonderful!