Do for her something meaningful. Do you know any movie that she would like, but she knows you wouldn't stand it?
Any shop she would like to go, rock concert?
You can always think of something cool that she likes, and not even necessary expensive. Try to look through your memory, since you love her, you must have it somewhere in your head what it is she would like. If she is the romantic type, buy her flowers, write a song, something of that kind.
If you know her to be pragmatic, do something more practical, but not too much.
It's not necessary buying, or even saying the right words. Sometimes it's as simple as finding a cool object, picture, or making a small sacrifice for your beloved.
And whatever it is, I believe you can find the answer. Your instincts ought to be good about the one you love.