[cryzinger]

Lego also has "3-in-1" sets that come with dedicated instructions to build different possible configurations out of the exact same pieces, which seem like a cool way to encourage kids (or anyone :P) to then veer off into their own building experiments.

[AlanYx]

There were also two 20-in-1 sets recently, both very reasonably priced. Those were huge hits in my house.