From experience, I have found that girls tend to like something you have written yourself, that is heartfelt- so give it a go! If you don't feel confident writing something about your feelings, then try and keep it lighthearted and funny. Even if it is not the greatest, most romantic piece of writing in the world, I guarantee that she will appreciate that you took the time to do it.
Another good idea would be to find out her favourite authors, poets, or musicians, and try to find a passage from something that they have written that reflects how you feel. Don't try to claim it as your own, as she'll probably recognise it! But as above, she will enjoy the effort you have put in by finding something that is specific to her, from one of her favourite writers.
