I really don't have any good ideas, but I'll tell you what I'm doing for my best friend's graduation (you guys know her as Denise, D or DsBlu01CivEX). Two of our other friends and I taking her to the beach. It doesn't sound like much and it's really benefiting all of us because we all want to go, but we're paying for her share of the room and probably lots and lots of drinks.
But I agree with the above ideas...you should take her somewhere or plan something special. Buying her something is nice, but sometimes it shows more and means more if it's something you can do together.