Well, you must be doing something right if you've been dating this girl for two years. This is a hard question to ask someone who does not know your girlfriend.

What are her special interests? Maybe you could do something to learn a little more about one of her hobbies -- for instance, if she likes to go running, maybe you could start training and run a race with her. Or you could make her a dinner of her favorite foods.

The most romantic thing you could do is something that takes into consideration not just *what* she is (your girlfriend) but *who* she is as well.