These Plywood Play Planks look like the coolest thing since Lincoln Logs, although on a much grander scale. When I was a kid, I had a little cardboard playhouse that we loved, but of course it fell apart after a while. I can only imagine how fun it would have been to build kid-sized wooden houses and other various structures with these play planks.
You can see descriptions of plans and materials from the December 1953 issue of Mechanix Illustrated at the above link. Maybe some handy carpenters today might like to give these a try!