Party is nice.
Think of things to commemorate the moment of adoption rather than traditional gifts of toys ect. They likely have all they need already in that regards since she's been living there for a while
Things as gifts
Collage of memories pre adoption
'Adoption book' instead of baby book
Jewelry (they make adoption type stuff, or even mothers pride/charm bracelet) for both the child and parents.
A 'coming home' outfit
Memory box (to put things commemorating the day in... Like the coming home outfit... Lol