Adam Saltsman (@ADAMATOMIC) created Canabalt in 2009 and ushered in the era of the one-touch control, endless runner genre.
The game's core mechanic is simply to run and jump from building to building and dodge obstacles that might slow you down. Your score is measured by how far you run before falling to your doom as the cityscape collapses around you.
Here is an excellent article:
http://www.gamasutra.com/blogs/AdamSaltsman/20100929/6096/
in which the game dev talks about how he went about "tuning" the level design and gameplay mechanics in Canabalt.
This week's Development Milestone is that by Friday you must have a functioning prototype video uploaded to your blog that demonstrates how the core mechanic of your game will be implemented using GameSalad. You should experiment first using simple geometric shapes until you have the core mechanic working properly rather than worrying at this stage about using your finished art assets. Take screen grabs of your progress and chart your process on your blog.