As an example of designing a game in a single day, I've built LEO Defense.
Currently, it has no real effects or models, just using primitive shapes to define the game objects. Green is for aliens, Red and yellow for attacks, and boxes for cities on the planet.
Another day would allow for some basic models, special effects, and polish to the gameplay, but I wanted to show a six hour day of gamebuilding, from concept to playable demo.
Controls are easy: Leftclick to shoot a bullet, Rightclick to fire a bomb.
There are limits to the number of bombs you can fire, based on how many cities you have.
See if you can figure it out. Link to the game: LEO Defense Webplayer