I'm a 13 year old girl myself.
Body spray/perfume is good. Vanilla, Cherry Vanilla, Citrus, and Cotton Candy are really good.
Jewelry... Claire's is a good place for this. They have earrings, bracelet, rings, ect.
Maybe some colorful pens, sharpies, ect.
A cd of their favorite bands that they don't have?
If they're allowed to hang posters on their wall, they can get some of those...
If they have an iPod, you can get them a cute case for it or a new pretty pair of earbuds.
A subscription to a magazine they like?
I'm completely out of ideas. Lol.
Edit: If they like to read, some good books are:
Chicken Soup For the Soul books
Gossip Girl (It's a little... mature, so watch out with this one.)