So Desert Bus is underway, and they're off to a really nice start, and I'm concentrating on that at the moment (and yes, I have donated), so I'm going to give you something rather simple.
Or, well, not so simple. It's a car engine. HowStuffWorks has a little beginner's tutorial on what exactly is going on inside that giant block of metal and hot spinning stuffness. There's a pretty simple game here in which car parts are individually laid out and you have to label them.
And now I'm off to continue looking at a bus.