The best toys are the simplest ones...

Remember Marble Run?  My daughter was looking for a present for her 5 year old step-nephew and chose this because she loved it as a child. It was an instant hit - both with the 5 year old and the adults. While the 5 year old was otherwise occupied, the adults could not resist building the hardest version in the guidebook.  The 5 year old was not impressed - either with the fact we had played with it without him or with the fact that we had not "used our minds", and after a few minutes of trying it out, he decided to take it apart and build something of his own.