I'm not sure about getting the room decorated. I surprised my daughter with a trip in October for her 16th Birthday. I used Pixie Gifts (Beth) and arranged for a cake and a goodie bag to be placed in our room. (Note--CS was not very cooperative in getting the bag delivered to our room--not their policy.) I had balloon's ordered and some Disney dollars for her to spend as she wished. They will customize a gift for you. Used them twice and loved their services.
One big thing--get your daughter a Birthday Badge (available at all parks). My daughter got one when we first arrived and wore it the entire trip. She was sung to over and over again and even at 16--she just loved it! On the bus, in the parks, at meals, etc. People would stop in the parks just to wish her HB. We were waiting for the boat at WL, and the driver made everyone stop boarding and called her over. He had everyone sing to her and told everyone the BD girl had to board first! She was somewhat embarrassed but loved it! I loved it more--to watch the look on her face was a priceless memory! Made her special day even that much more special. WDW does Birthday's right!
Also, we had a CM in Frontierland ask her if she was sung to by Goofy for her Birthday. She said no. Before we knew it, the CM picked up the phone and dialed a number and handed the phone to my DD. It was a tape recorded greeting from Goofy. She actually blushed!!!! It was awesome and a lifelong memory.
Make sure it's noted on your ressie that it's her BD as well as on all your PS ressies. You can order cakes for around $10 at some of the restaurants (I know LTT had one). They then greated her by "name" (she did not know how they knew her name) and had confetti and steamers on the table. This was so the characters knew it was a "special day" and they spent extra time with her! She loved it. They even had the CM's sing to her at the end of the meal.
Hope this helps!