If your girlfriend likes pasta, you can't go wrong with that. It's inexpensive, it doesn't take long to put together, and it's hard to mess it up.
With a loaf of garlic bread, salad, and a couple glasses of wine (red if you use a tomato or meat sauce, white if you use pesto, olive oil, or alfedo sauce), that will make a very nice meal. Furthermore, your girlfriend will appreciate the effort, even if you didn't really put that much into it. (Because pasta dinners are, after all, easy.)