Look at her with love in your eyes, and hold her hand, and listen to her. That's way more better than rose petals shaped in the form of a heart.
Rose petals are always great, but add some candles - particularly votives so you don't have to worry about spilled wax or fires. ;-) Other than that, you know your girlfriend better than anyone on the internet. What are her tastes? What kind of music does she like? Would background music help or hinder? How nice of of you to do this for her. :-)