My good friends and I have had some great fun just wandering the shops. We stop for coffee, buy things for our families, talk about anything and everything, make each other try on ghastly outfits and laugh our heads off. She might just appreciate a nice, low-stress, afternoon out, talking, visiting, and laughing. And, you might notice something that she really likes, that is not too expensive, and you can surprise her with it at the end of the day.
Otherwise, if you do have a picture of the two of you, you could have it framed nicely, with a large, light coloured mat, and then write what ever you like on the mat - a good story of something the two of you did, things you appreciate about her, a few of your best memories...etc. Or simply write things like "best friends" and your names in stylized letters, with flowers and swirls and stuff like that to decorate it. (I can picture it, but can't describe what I mean too well - maybe visit a scrapbooking store for a good idea.)