I finally let DS have at the lego collection my sisters and I had when we were younger. The first thing he does in the morning is dig it out. As soon as he is home he is playing again. I'm continually getting pestered to "help" i.e. make things from the "constructions" he's found in there.
I surprised myself when I did actually sit down with him and make some stuff up - remembered how much fun it is to build things.