Once you have a simple game code working, it is time to start the wonderful world of hardware hacking. The joy of the Pi for me is that it is able to interface to hardware. I've just started dabbling in the electronics side of things (I got an arduino kit a month ago and am amazed by how easy it is to build stuff that just works which has led to increasingly advanced projects and following my curiosity.
I'd take something like Hunt the Wumpus and make a simple hardware controller for it. Left/right/Forwards etc. Maybe a Wii Nunchuch could be used.
Once we can get people into the 'I can build a beam splitter that will count how many times the bird goes into the box' or similar, then real physical things become far more enticing than virtual ones. And modern semiconductors are really cheap.
the Uni at which I work has a quote from one of it's fomer principles - "By creating we think, by living we learn". Taking what we see and copying, then extending. By doing, failing, repeating, succeeding we learn.
If the RPi can encourage that kind of curiosity in kids and adults, the 'I can do it' attitude, then it will be a resounding success.