Fun but appropriate Christmas gift to give influential professors. Inspired by this question...
Background
A friend and I have been meeting informally with a retired professor to do math on a weekly basis for several semesters now. He is a dear mentor to both of us. We would like to get him a Christmas present, but he is a very private person and rarely discusses his home life, so we know almost nothing about him outside of his mathematical interests. We could get him a book, but chances are good that he would already have it. (His library is enormous, and he's always bringing in obscure texts.)
Some possibilities:


*

*A copy of The Emperor's Club? This is a movie about a teacher and his influence on his students. I'm worried that might be a bit cheesy or inappropriate.

*Something mathematical, but more "popular" that he is less likely to have, like a film or a mathematics-strewn crime novel. (No textbooks!)

*A pie with a $\pi$ carved in the crust.


We're really stumped.
 A: If I might make a suggestion. 
I don't know the man, but the heartfelt words you expressed above say it all. If it were me, I would love to hear that from someone I've helped. 
Perhaps finding a wonderful inspirational quote and then personalizing that with both your heartfelt sentiments would be worth gold to someone as they feel like they continue to pass on their good deeds! 
I would even go as far as making it an inscription on a plaque of some sort. I would cherish such as thing forever and would display in a place of honor if I received that.
Both of you young people give us all hope!
"What you have been obliged to discover by yourself leaves a path in your mind which you can use again when the need arises." - G. C. Lichtenberg
Regards -A
A: This is actually a serious suggestion. Math and Sex has always been loved by everyone I have given it to. Nope there is nothing inappropriate in the book but the title does get everyone's attention. It is my favorite gift to give. I have given a copy to everyone.
A: As a bit of shameless self promotion, might I suggest a randomly generated book, produced by Mathgen.  It's unlikely he already has it!
(Full disclosure: it's my project, but I don't personally profit from book sales; all my proceeds are donated to the AMS.)
