Well, it depends.. If I pick up a classical guitar, I'd play some Mozart or I know what on it.. Also, Classical Gas..
If it is an acoustic, I'd do some David Crowder's "Make a joyful noise" riffs, and then some funky riffs of mine.. Along with some Shane&Shane stuff..
If It'd be an electric. I'd play here and there a little solo and a chord or two, because I'm not that much into electrics yet.