I'll start off with one.
(If you have good at mapmaking experience) Create a Starcraft UMS movie map that tells the story of your relationship, or the story of one moment with her you really cherished. Play it out with the right units, texts, and graphical effects. Tell her to log onto Starcraft and join your game and watch your UMS movie that you had created with you.
Now share yours. I'm in need of original ideas, and I'm quite sure alot of you with gf's may benefit from these ideas as well. Of course they don't have to be starcraft-related (I actually encourage that they are not).