We are going for my daughter's 16th, and she is bringing a friend with her. They plan to do some Disney Bound outfits for a few of the days.
There are a lot of free activities too, like movies on the beach at night. We love the Chip and dale campfire at Fort Wilderness, and it is easy to get to from Magic Kingdom by ferry. They could toast smores, enjoy the movie, and get pics with Chip and Dale.
I haven't decided yet on getting a cake, but for her 5th birthday we went, and I got a cake at Publix that was really good. I think it was just as good as the Disney Bakery cake I ordered for my dad's birthday one year from the GF.
You could also decorate the room with ribbons and streamers while they are out at the pool.
I like the idea of having characters sign a photo frame, might steal that one.