Go play Naya’s Quest.
It creates very interesting puzzles based on simple mechanics, and does a great job or remixing on top of itself toward the end. The New Game+ lets you see behind the curtain as a reward, rather than just making the game more difficult.
Here’s a little retrospective from the development of Spore.
On my last project we leaned heavily on rapid prototyping, and while there was a fair amount of throw away it did help us ‘find the fun’. It is easy to spin and burn time heading down blind alleys, but keeping a focus on the questions you are trying to answer helps immensely.