Easy generic answer, something thoughtful.
What sort of things does she like, music, films, books, clothes, food etc etc
Once you've thought about that, try thinking of things relating to that which she might really like, and sometimes think a little outside the box.
For example, if she likes clothes, you could go to a shop find something you think she'll like and buy it, making sure to keep the receipt incase she doesnt like it. Or alternatively, you could design and make a voucher and a card detailing a day out shopping together, as your treat to her. You could make a day of it, start witrh breakfast in bed or something, then go shopping togethe, making sure her shopping is the priority so you go to all the shops she wants, and you don't complain or anything. And treat her to however much you want to spend. She can always spend some of her own money too. Then take her for lunch. In the afternoon continue shopping. Then in the evening cook a nice meal and have a relaxing evening in.
Girls love this kind of special attention!!!

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Valve