I agree with many of the suggestions made so far. My personal
suggestion is IceTowers then Zendo, because they are both so wildly
different in the skills required to play them well and how they use
the pyramids. After learning both games, nobody will look at a set
of Icehouse pieces and think that they can only be used one way for
one game. I'd then look at Playing With Pyramids to see what to hit
next, based on your own preferences.