Gameplay Feature: Game within a game

Games featuring games within themselves. Usually these are played by the protagonist (not the player) and available as (hidden) collectibles/unlockables.

Limitations

 * Some games are a collection of minigames themselves, sometimes called stages (e.g. various olympics games), these don't qualify here.
 * Some games launch a minigame (i.e. distinctively different from standard gameplay) for special activities, such as picking an electronic lock, these don't qualify here.
 * Special levels (e.g. beer collecting in Tyrian) don't count here.
 * For games that let your protagonist to gamble within the game universe, see Gameplay Feature: Gambling or more specific Gameplay Feature: Poker.
 * There is a separate group for games which include previously released full games (hidden/unlockable): Games with hidden / unlockable full games

Examples
System Shock 1 & 2 has games that can be collected in cyberspace and later played, Anachronox