Well, one of my best friends is a female, and on her last bday i gave her a giant fluffy teddy bear in pink trust me she loved it, and by the next day all her closest gfriends knew about it My advise - do the same, you won't regret it
Maybe get her an ITunes gift card, or a gift card to a store she likes. But if you want something more personal (and have several weeks) then you might think about making a scrap book with mementos and pictures of the two of you or something.
tickets to see a musical always go down a treat if they are into that sort of thing
jewlery but depends if you want to go out with her or not after this cos i would say jewlery is a gift you buy for someone you are going out with or want to in the near future
if you wanna go out buy her a heart shaped locket and have her name engraved on the back she will love it this is what i bought my gf for her birthday and shes worn it everyday since