The geekiest thing I've done in my life was, while in college, staying all saturday morning, afternoon and partially night at a computer lab programming a compiler (actually an interpreter) from scratch. Than was very geeky. But it wasn't really in public, so, let's see...

Arguing with a multiplex manager about SW release date problems in public was very geeky, now that I think about it...
Playing RPG at a mall food court with people looking at us screaming and laughing like maniacs... That was embarassingly geeky.
Going on a insane shopping spree at an anime convention last year...
Going to several anime conventions, RPG conventions, B-movies conventions...
Oh, and going to a 007 fans convention, that was pretty messed up, I don't know why I did this... And I was, like, 12 or 13.
Quoting unknown, obscure anime quotes at business meetings and stuff.
Quoting Star Wars in several ocasions, ocasions which I should NOT be doing that.