Girls at times are super hard, having raised two of them things change from day to day, or moment to moment...
I always went with what I knew that they loved or liked all the time -
Something Disney - Hoodie, Socks, T-shirt, sleep pants, or night shirt -
Bath products, Make-up, Hair stuff, Nail polish/nail kits -
Make-up bags, and other little bags that could put personal items in, or the one's that could be attached inside of a dance bag, or whatever else that they thought of.
As far as clothing, this changes constantly as they are figuring out their own style. So I stayed with jeans, solid shirts and sweaters. Not unless I was 100% sure they would love it and it just screamed them.
New back pack, and school supplies
Flip Flops - living in Fl, flip flop are a way of life and you can never have to many pairs...
Bathing suits, beach towels, beach chairs, sunscreen, and bug spray - again living in FL this stuff we use year around.
GC... to their favorite spots...