I remember never being able to do Rubik's cube when I was a kid! Not sure now how old I was when they came out. Is there a slighter easier version of that type of thing? (Kids are 9 and 5, albeit better at puzzling/spatial stuff than I am) Maybe the snake?

Any little science doohickies like prisms (good idea) are also great, but they have to work and not be lame/chintzy...you know how that can be...