I'd get something tiny on my ankle... either an Anglican cross or a treble cleff. Those are probably the two things that are most important to me - religion and music. *rethinks most important things* okay maybe a Harry Potter spell in really cool caligraphy.
I have a few I'd like to get done eventually. The first two are really sappy. :P
I want to get a half a heart on my right hand (my girlfriend is going to get the other half on her left, so when we hold hands, it's a heart), a tattoo of a ring on my ring finger (to go under an eventual wedding ring, again, girlfriend will do the same), the word Love in a fancy script on my right shoulder, two bird silhouettes on my left shoulder, the treble/bass clef heart on my hip to the right, a Fibonacci spiral somewhere, and a third eye somewhere.
Lol... just make sure you and your girlfriend are really commited. You don't want a tattoo on you that reminds you of her every time you look at it if you are no longer together. Not that I assume you won't be together forever, I have no idea the status of your relationship. It's just that my friend and her boyfriend did something similar and they broke up two months later and she's regretted that tattoo ever since. :-)