My ds will be 4 in November. He loves duplo (we have it all) and Thomas the Tank ENgine stuff (we have a huge amount of Brio/ELC stuff) and he is great at construction. I need to think outside the box. My dh thinks he's still too small to move onto real Lego (maybe he is right) so what else is there that will really challenge him and at the same time not be too difficult? He's good at building, and I've heard about K'Nex. Any thoughts? Also wondering about things like Scalextrix/Hot Wheels. Any thoughts? Thanks