I already figured that with a week remaining, the game'd have to be simple - not simple to the extent it'd be unfun! Some of the most popular game concepts derived from old arcade games. It was the arcade, retro style I wanted to pursue.
Helicopter
Helicopter was my initial idea; it has a great concept that's fun time-and-time again. The game is based around a side-scrolling cave which you have to fly through, but it has a twist. Your helicopter's failing and there are obstacles in the way.
The helicopters position has to be righted by clicking the mouse button. The game is based around up and downward movement to get as far as you can by dodging obstacles; if you hit one, it's game over and you have to start again! Old school.

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The original Helicopter can be played at http://www.helicopter-game.org/.