When I got my braces off at 12, my parents decided to get a new "family portrait" When I saw the pics back with my smile, I felt like a brand new girl. Here's a suggestion, if this girl was really obedient with the food rule with braces, and stayed away from all things sticky and chewy, maybe some twizzlers or something?
Get her something she wasn't able to eat when she had her braces on, like gum for example. But other than gum get her something good something she really likes but was not able to eat because of braces.
Whatever she loves. My parents got my brother a huge chocolate bar from Krause's Homemade Candy when he got his off. I haven't gotten braces yet but will be soon. :/ Hope this helps, good luck! ~Alicia