For my Sweet Sixteen, I got a variety of gifts. I got a lot of gift cards and money. One of my best friends got a GIANT picture frame and framed a picture of us. She then wrote ALL our inside jokes on it. One of my other friends bought me a pair of my favorite jeans (J&Company) and wrote all our inside jokes with metallic gold pen.
Three of my friends did a joined gift and got me two Juicy purses and a Juicy sweat set. I really love those pants. Maybe you could do that? Join in with a few friends and get her something nice, but you know she will love.
If you know she is getting a car, ask her parents, you can get her some accessories for her car. My friends bought me an ipod accessory that lets me listen to my ipod in my car. Maybe make her a mixed cd of her favorite songs so she can play them in her car.
Whatever you get her, it should be personal. You know her best...good luck!