Adventure is often voted the best Atari game ever and is usually included in lists of the most influential games ever because it was ground breaking for so many reasons. It was created by Warren Robinett.
Robinett combined the fast, reactionary gameplay of action games like Combat or Space Invaders with the exploration and puzzle solving gameplay of text adventure games like Zork to make the first "Action Adventure" video game. When he first described the idea, he was told it was impossible and to not work on it, but he did anyway.
Almost all video games, and definitely all Atari games, took place on a single-screen play field. Robinett invented a mechanism to switch the screen and make it appear like you were moving through a world much bigger than the Atari 2600 could render at one time. We say first "game engine" because technically it is not the first game. Atari Superman reused Robinett's code to make a multi-room game and was actually released before Adventure.
For the record, developers have been sticking weird, hard to find surprises in video games for as long as video games have been developed, from a McDonalds on the moon to a way to get free games. Robinett was not the first by a long shot and can technically be called the first only because his was the first time the term "Easter Egg" was used to describe it.
But Robinett's easter egg was seminal in that is was invented not just for amusement, but as a statement for game developers who were making millions of dollars for the game companies but not being allowed to take credit for these games. He implanted his name secretly in the game the same way an artist signs their pictures. The effect was multiplied by the mass availability of Adventure, which was played on a scale far greater than previous games. The easter egg was discovered and spread by word of mouth until one child mailed Atari and told them about it. This created a board room eureka moment for Atari when they realized that rather than actively discourage these surprises they should, instead, encourage them to promote further interest. And the video game easter egg was born.
Yes, Warren Robinett was trying to create Head-to-Head Atari Adventure back in 1979. He wanted a two player game and created a cable that could connect two Atari 2600 consoles running on two separate televisions. The cable would go from the 2-player joystick port of one 2600 console to the 2-player joystick port of the other so that the consoles could sync the players moves and game state. He created and tested the cable successfully, but, as he put it "I was already out of memory and I had one foot out the door" so Adventure was shipped as is.
Just imagine if Atari had shipped a networked version, and your friend could bring over his Atari and play a whole new version of the game. Think of how Atari could have leveraged its already dominance in the market and perhaps usher in a whole new generation of games. This was nine years before the networked, home video game, MIDI Maze, was released on Atari ST, which did not have nearly the same prevalance as the 2600, and so networked games did not start to drive a new generation of games until the internet became widely available in the late 1990s.