I agree in that you know your sister best and can choose according to what she likes. I don't think you can make a bad choice at WDW. Be sure you tell them when making your ADR and when you check in at the podium that you are celebrating a birthday. Check out the Birthday Cake thread to choose a cake and order it accordingly. And last, but not least, don't forget to pick up a Birthday Pin at Guest Relations in whatever park you are visiting.
Whew...with that all said...we are celebrating my niece's Sweet 16 with breakfast at 1900 Park Fare (She LOVES Mary Poppins)with a miniture wedding cake with a castle top, a day at the Magic Kingdon, dinner at Ohana, her favorite, with a huge chocolate cake, followed by a night at
MVMCP.