I'd like to nominate the regional FO I saw sitting in the crew room (presumably on airport reserve) who had his laptop and joystick set up, and was doing a MS Flight Simulator version of one of the routes we do in the real world.

Making it even weirder, I'm pretty sure he was doing this for a "virtual airline", so he was simulating being at work, while he was actually being paid to sit and do nothing at work.