I surprised my DD for her 16th Birthday in October with a trip to WDW. Here are a few suggestions:
1. Definitely get the Birthday Badge available at guest relations at all parks. I cannot tell you how many CM's and guests wished her HB. She was just elated. It was so cool to see her blush at 16 from all the kind wishes. The boat captain told her she had to be the first to board the boat to MK at WL because it was her BD after everyone sang to her. She was sung to on buses, in restaurants, etc. Memories she'll have forever and so will I.
2. I ordered a gift package from Beth at Pixie Gifts. I've used her many times and their service is just awesome. I cannot say enough. I had a BD cake, balloons, some t-shirts, night shirts, frame, etc. waiting for her in the room at check in. They are a little pricey but it was worth every penny. I even had Disney Dollars in the gift bag for her to spend whatever way she pleased!
3. Make sure it is noted on all your reservations--hotel, dining, etc. Some restaurants you can order a small cake for around $10 or so. We did PS at LTT and because they knew in advance it was her BD, they had a special table all set up for her with confetti, etc. They immediately addressed her by her first name and she couldn't figure out how they knew! Because there was streamers, etc. on the table, the characters made a point to stop by often to wish her HB, etc. The CM's sang to her and brought her out a complimentary cupcake. It was cool.
4. One of the CM's in one of the gift shops actually called Goofy and had Goofy wish her HB on the phone. They have numbers they can call and there are pre-recorded messages. She actually blushed--it was so cute!
We really enjoyed her Birthday trip. We both have long lasting memories. I hope your daughter really enjoys her special day at WDW!