Forget all the fashion ideas. Teenage girls like music (a music gift card might be a great choice), video games, and age-appropriate books such as the Twilight series. They like to be creative: how about a blank book to use as a diary/poetry/picture collection place.
Teenage girls tend to like anything involving fashion, makeup, or pop culture. Though those who know her well would know best, I can give a few general reccomendations. A subscription to a magazine such as Cosmo Girl or Seventeen would be nice, as would a handbag, nice beach towel, or even a nice shirt.
Teenage girls like a wide variety of things, at the age of 15 they are starting to want to be more grown up. As a result I would go with something that is monogrammed, or maybe even a Vera Bradley bag. In addition you could go with troll bead jewelry.
Teenage girls like anything designer, it is usually a time when they are trying to impress their friends. Good examples are Hollister or Abercrombie and Fitch clothing, Tiffanys and Co. Jewelry or Juicy Couture. You can find good deals at any of the above mentioned.
teenage girls like new dresss which would be so casual and attractive. teenage girls also like interesting show pieces which are related to some theme such as love,freindship etc