I would suggest putting together a book. Send out a piece of stationary to each family member and have them write memories or special things about your daughter on each piece of stationary. I would suggest using the same paper they write on, as the pages of the book.
In high school I was a peer counselor and on the last day of school, we all had to do this for each member of the class. It has been something that I have cherished always, knowing that I had so many people that loved and supported me.
One other item that might be special is a GEODE rock. I am not sure if you know what that is, but it is a rock that is nothing really special on the outside, but beautiful on the inside. You can write a poem or card to go along with the rock explaining how beautiful your daughter is on the inside and that no matter what she looks like, what mistakes she might make, flaws she might have or what anyone ever says about her, she will always know that she is beautiful on the inside, NO MATTER WHAT! Just like the rock that has flaws and imperfections on the outside. This is something that I had a Psychology Professor tell me and then he handed me a geode and it is still to this day, ten years later, next to my bed! Good luck and what a wonderful mother your daughter has!