alt Hacker NewsBlocks is the top comment (for me); and yours is number two. Timeless classic. Another one could be plasticine clay. These toys afford play, they don't direct, restrict, or guide play. Other good toys like this: box, stick, the woods, paper (especially a big roll of butcher paper), and things to draw with (I find a black, red, blue, yellow and green sufficient).
I recently needed to make a mockup of something, so I got some plasticine from Amazon, since I remembered playing with the stuff when I was a kid. However what I received was quite stiff and left an unpleasant oily smell on my hands that I had to scrub off with a lot of effort. Is there a particular brand of plasticine that you have had a good experience with?