My little sister is in this age group, so I'm getting inspiration from what she likes.
Art supplies maybe? Paints, a sketchbook, drawing pencils.
Beads and beading supplies, this is something fun that she can make and wear, and even share with a friend. I'm 26 years old and I still enjoy sitting down with my mom and my sisters and making jewelry.
Perhaps a cheap digital camera? Funky socks (knee high, striped, sparkly) and fashion accessories are usually a winner and even result in squeals of happiness.
Personally I am not a fan of Seventeen or any fashion magazines as I think they are full of unrealistic beauty ideals and models that are so airbrushed that they are impossibly perfect and borderline inhuman, but that's me.
If she likes to read, books! Or a gift card to a bookstore.