Would you happen to know what size pants she wears? Levi's jeans are still a very big hit in Spain and they are very expensive there. I think that would be a good gift.
Otherwise, if she is learning English, I would give her an easy American book that would give an idea of the American culture like "on the road" by Jack Kerouac or anything by John Steinbeck. Cheesy American magazines are good too, like People, the National Enquirer, etc....
Also, what was popular in your school? Like those yellow bracelets for Lance Armstrong. Also, any type of American foods (esp. junk foods!) are a novelty as well, like moonpies, kool aid mixes, root beer, cheese-its, and Reeses pieces.
Even if she might find these foods horrible or maybe she'll like them (or at least be polite about it), it will be a good story to tell about the food and then esp. experiencing it with her.