DS is 3 and loves robots. He also loves taking things apart and putting them back together - he was doing it yesterday with a clicky pen, reduced it to a pile of bits then spent a happy 15 minutes working out how they fitted back to make a working pen.
Lego isn't quite what I'm after - he does have Lego but prefers to just build at random with it, he can't follow the instructions yet. More like a toy with interesting fiddly parts, that comes assembled but can be disassembled and then reassembled...?
What do I want to look for, anyone? It doesn't have to be a robot I suppose, if there's something else that fits the rest of the criteria!