Gift for mentor after his thesis defense? I'm attending the thesis defense of one of my undergrad mentors this week. I owe almost everything about where I am today to him. I want to get him a gift, but I'm not sure what. He owns his own business, so a gift card seems pointless when he won't be short of money. I have never seen him eat or drink anything, EVER, so I'm not sure what kind of candy or food would be appropriate. He's a guy, so flowers are out. 

We did have a shared love of a certain book series, but I already got him one of those for a Christmas gift and I feel like it might be weird to get another one again. 

His thesis is something pretty abstract—even the name of the project has no real-world analogue. So a "cute" gift related to his thesis would be tough to come up with.

Also I'm a girl, if that matters, and I don't want to give him anything that would be creepy/weird/over-the-top with the gender difference taken into account.
A short (1-2 paragraph) hand written note on a card or postcard would be an appropriate gift. 

I have mentored about a dozen undergraduates and I still have the thank you notes they sent to me on my pin board in my office. The notes are nice reminders about the impact I had on the students.