In my experience, there is a sort of tradition in science that a supervisor gives a gift to a trainee leaving the lab. I think it's a nice gesture and I know I appreciated it when I left the various stops in my academic life. Of course, now being on the supervisor side of things, I've got to be the one coming up with the gift ideas.
I bring this up because I have a student graduating by the end of the summer and since my summer is a vortex of deadlines and travel it occurred to me that I should consider what I would get as a gift now rather than picking something up at the local convenience store 10 minutes before the defense. I mean, every likes fudgesicles, but they may not make the best going away gift.
The fall back for almost every supervisor is books. We all like books and there is an essentially endless number from which to choose, but unless you know what the person leaving is moving on to, picking the books that will be useful to them in the future is not all that easy. Plus, at the rate people move in most academic fields, you might as well be giving someone lead bricks.
Surely there are more innovative ideas out there.