You're going to have to tell us a bit more about the woman in question before anybody can really give you a sensible answer.
What does she like? Music? If so, what kind? (itunes giftcard, perhaps?)
Movies? Which type? Get her a DVD of a movie you know she loves, but doesn't yet have.
Books? Same story.
She might like being taken out on the town. You know her to some extent, hopefully, so your the best judge of whether she's more 'taken toa fancy italian place' or 'get some bad chinese takeout and watch a movie at home'.