How about Cowboy hats and spurs?
Hmm, no? Well, I think good alcohol is always a hit (I hear whisky is pretty popular, but maybe look for something they'd have a harder time getting, or even learn how to mix a couple drinks). A couple other small 'american' things would be good. Clothes? Maybe things like shirts/sweaters from your university or something.
haha, Do the parents know she's your girlfriend? And they're letting you stay at her house? I imagine they're "fairly" liberal. So atleast you have that to go on.
Later,
Stets.