Elizabeth, I like the idea of something personal that would have meaning to both of you such as Joy's suggestion of a small framed picture of the two of you when you were younger. I turned my mom's house upside down trying to find a photo of me and my best friend (taken when we were five) for her 50th birthday. I never did find it, but it would have been so sweet. We were standing on a train platform holding hands during a school field trip, wearing our best coats and engineer caps. Maybe I can try again to locate it for her 55th.
Another idea might be a Forever Friends necklace such as this:
http://gifts.redenvelope.com/n.....Hribbonlad
I love the inscription on this necklace.
Or a personalized purse mirror (I actually give these for graduation gifts)
http://gifts.redenvelope.com/h.....Hribbonlad
http://gifts.redenvelope.com/h.....Hribbonlad
I hope you have a wonderful time with your friend!