Today we visited a place called Adventure Playground. It's a wooded park where kids are given tools, scrap lumber and hard hats. Then they're pretty much given free reign. (This is easier for some parents then others of course.)
It was awesome. The kids wandered around, checking out the other tree house type structures and added a bit here and a bit there. My kids haven't done much woodworking or building type stuff so they had fun just pounding nails for a while, and carrying lumber around and dreaming of things to add to the structures.
And yes, I had to just swallow back some of my advice and concerns and let them figure things out themselves. They didn't end up building anything significant, but they had fun exploring, and that's the whole point, right?