Does she live on her own? With you? etc.......
Make her a tape of romantic songs that you both listen to and have meaning (if you guys don't have any or alot, then make some that you think she would like and would have meaning), put it in a box along with a card and then go to La Senza and get her lingerie. Tell her on the card that she's special and means so much to you and you couldn't ask for anything more in a gf and you are looking forward to seeing her that night. Drop the package in her mailbox when she isn't home so it's waiting for her when she gets there. You'll hear back from her within minutes of when she gets it.