something that a) takes effort to acquire/create, b) something that comes from the heart.
back when i had a gf, for her birthday i made a giant collage of pictures of just me and her. it was taken from hundreds of pictures that i cut out, and pasted onto a giant bristol board, and then i had a little quote on the middle of it all. something that was meaningful to us.. know you.
sure, it's a little bit sappy, but anybody can go out and buy something. take the time to come up with something that really shows her that you care. it may seem a little silly, but the meaning behind your actions are what's really important.