For my entry, I decided to make something nice and simple... And this is what I cam up with:
It's a wind-up motor inside a 6x6 box, that moves the legs of the Technic guy, creating the illusion that he is pushing the box.
Video:
As mentioned in the video, I have also made instructions for this, which are available for free on Rebrickable.
POV-Ray render:
Some LDCad animations - rendered with POV-Ray: