Unlock Centipede Mini-game
Finish the entire game to unlock the Centipede under the Atari Games menu screen. If you don't see it at first, restart the console, reload your game, and then it should appear.
Missile Command Mini-game
On level 13 of the vet's office, walk to the small office on the far right. Go to the computer then hit the action button to play Missile Command. Complete the level to unlock the game at the Atari Games menu screen.