What should I do for my supervisors birthday?

I am currently participating in a program that involves spending months at a time, non stop, with the same group of people. One of those people is my supervisor. Because we have been living together for months, its not the kind of relationship you would imagine. He is my supervisor but he is also a very close friend. Because of the line that he can’t really cross though, he has been very lonely because he is our peer but also our supervisor. I think its a difficult situation to be in. I want to do something special for his upcoming birthday to show our appreciation, but he is a bit of an introvert and definitely wouldn’t want a party.. Any ideas?