As part of the process for making Wired, I worked with some students over the summer to create a load of gameplay prototypes.
Nearly all of these games took one person 2 weeks to make (though a couple took longer). They all have something interesting about them and are also all flawed to some extent. However, each prototype showed enough to know whether the core idea would work and where the potential risks lay.
You can play the actual games here:
http://i-want-to-study-engineering.org/gameslist/
Diarmid Campbell 