If you must know, instead of anything particularly clever, and under the piercing gaze of a BIG security guy, all I could manage was "I have no camera" (since everyone else in the place seemed to) "but thanks for eight great years."

He replied "Thank you, sir. It was an honor."

Actually I've come close to meeting him before, at political events, and have heard him speak, in person, at least two or three times.