Well...ok I am a bit of a big sap so this may be too much cheese. And it really depends on how much you like the girl...and how much effort you want to put into it. But "homemade" gifts, to me, are the better option in such a case. Now I'm not saying make something really tacky or completely from scratch....but....
...does she have a fave poem? You could write that up, or get it done in that fancy calligraphy (spelling) writing. Or you could be real adventurous and attempt to write your own...
....a mix cd/tape/whatever is great. But really make sure to personalize it, make it related to things - e.g. songs you both or she likes, songs that have some sort of connection, etc.
...and, without knowing much more about things, this would depend if you are still wanting to stay in touch (i.e. dating etc). A web cam (get your minds out of the gutter
) may be good here. OR maybe you can contribute to getting her or you some sort of long distance plan so that you can always stay in touch etc.
....maybe something from a place that means something to you both...whether you buy it or just get it. Could be a picture of it, maybe, framed. Or a souvenir, or something from it, or...?
Anywho, those are some random ideas...I'll add more as I come to think of them.
I guess for me the question is: is this something that you want to say "bye bye, I really enjoyed my time with you, but I'm not going to ever see you/talk to you again", or is it a bit more serious/let's stay in touch? I'm assuming its romantic? Or is it just a friend? OR....?
Cheers.