What does she like? If she likes music, get her a CD or iTunes gift voucher or a music book if she likes to play. If she if very feminine, get her perfume, or some sort of make up. Just don't get her a gift voucher. Girls like when boys put thought into what to get her
As a woman, I have to tell you that "expensive" is not always better. If a gift has meaning and is from the heart, she will cherish it. For me, simple time together is gift enough.