What hobbies does she have? What kinds of things do you like to do together? Can you snoop around her house to see what stuff she has? My bf and I don't really do presents for bdays: we'll usually go out for a nice meal, but he's taken me on a rollercoaster for my birthday, once, and a few years ago we went to Ireland to see Radiohead for his. It's the planning that goes into it that makes it really special.
I really like practical presents, but I don't think everyone is the same about that (I got a handheld food mixer for Christmas last year, and eco washing balls before that, so I think maybe I'm abnormal).