I think the author of Simplicity parenting might address the indoor/outdoor issue by talking about the abundance of stuff typical Ameican kids have inside their homes/rooms--generally plastic crap and way too much of it that can only be played with in very particular ways. Kids aren't making forts etc. inside because they are too overwhelmed with all of the stuff inside. When we remove the excess and allow them unstructured free time they can be creative in that way inside.