I really hope the adventure playgrounds mentioned are still a thing. The ones in Germany were a bit more organized than just a pile of junk, but you basically went there, maybe parents paid some (nominal) fee or deposit, and you got a toolbox with basic hand tools like a saw, hammer and nails and there was plenty of wood to build with. Larger structures were clearly built with some guidance and probably a civil engin…
Saying that where I grew up there were a lot of homes being built so there were piles of scrap wood, etc all over and we'd raid them and build forts using saws and nails from our dads' workshops. So maybe we are overprotective today.