For a "serious" game, which implies more then e.g. guessing the next random number the computer will generate or some simple ascii question-and-answer game, i think it is difficult to get simpler then pong. Although one of those where you display an image randomly and the player has to shoot it with the mouse could be even simpler (maybe). I was also thinking about the simplest game possible when i made my "plan" because even the simplest of games will include a main loop, some kind of input and maybe some kind of graphics, which is already a lot of topics. But these topics can be introduced before an actual game is made, there are even tutorials around here for most languages for displaying an image, reading the keyboard etc., so if pong would follow from these initial tutorials, i think it would be fine.