One of my favorite mementos from undergrad is a book from my favorite professor; when he died, his widow brought all of the books from his office to the on-campus memorial service and said that his students should take one to remember him by. I was pleased to see that one we had worked with extensively in his class was still there.
I vote book.
What a nice gesture.
I've given my students the following, depending on their interests:
Books. Giant Microbes for those interested in microbiology; related to their research. To a student that went on to teach: a carved onyx apple. A carved Buda from our fair trade co-op to a student who was double majoring in science and philosophy and had recently discovered Eastern religions. But these are students I knew well, so it was easy to give them something personal.