You could give her a basket of stuff that is sentimental and fun.
For example, you could give her a picture frame of the two of you - like the others suggested - that is engraved with a special friendship quote. A bottle of champagne or whatever she likes (danzka's my new favorite) to toast her new endeavors, some stationary to write to you, and maybe something that she will need that has to do with your major (like you suggested).
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