Adventure – designed and programmed by Warren Robinett and released for the Atari 2600 in 1979 – broke new ground at the time, on a number of different levels.
Not only was it one of the first ever video games to use multiple screens, which you could explore at will, but it also featured enemies that would continue to move, even if they were not on screen. It was also one of the first games to feature an “Easter Egg” – a secret, hidden room containing text crediting Robinett for creating the game.
In Adventure you basically play a square dot as an avatar, and must explore an open-ended environment in search of a magical chalice. You can pick up keys, weapons, and other items and must use them to solve simple puzzles. A number of dragons roam the game world and will eat the character if caught. A bat will also steal items from the player too, so has to be avoided.
Looking at Adventure now: it would be easy to dismiss the game due to the simplistic nature of the graphics and gameplay, although its influence after release was actually quite profound.
More: Adventure on Wikipedia