The Menu Room Has a Working Text Adventure Hidden Inside It
Before you even hit start on the campaign, you're strapped to a chair being interrogated. Rock the triggers back and forth fast enough and you break free, wander over to the terminal in the corner, and can type in a bunch of commands. One of them just loads up Zork, the 1977 text adventure Activision happens to own the rights to, complete with its own achievement for getting eaten by a Grue. It's a proper, playable version too, not some stripped-down tribute.
