What's age-appropriate? The nails were too big to swallow and the hot glue guns the kind they sell at CVS. The kids cut and poked themselves and learned to be careful.
At summer camp when he was 8 he learned to weld, braise and solder. 12 kids aged 8-10, one instructor. I saw a photo one parent took: instructor helping kid hammer something on an anvil; behind his back a couple of kids kneeling and welding.
Another story I love: when he was 12, my son and his friends found bottle of vodka in the schoolyard. Being 12 year old boys they did the obvious: filled on of the kids' sneakers with the vodka and set fire to it in the schoolyard. Apparently the math teacher walked by and called out "Boys! Class is in 5 minutes!"