Used to sneak out onto the roof,
Amused ourselves with a home- made air rifle, which was a length of pipe, fridge compressor, washing machine valve, switch as a trigger..and sights. Reached 60-70psi - stored in a plastic coke bottle. This thing ruined our fences with pieces of bolt, but was more fun firing dried peas which used to spiral 30 feet before shattering on impact. Fun that was until my brother shot me with it. Fortunately just a pea, but still had to be dug out. Oh - and it was a 240v live supply.
With the application of a few caps in various small munitions this thing could punch holes in steel plate.
Chemisty sets. Real chemistry with some quite toxic stuff, and some that was more fun I'd rather not talk about. Made rocket propelled things like a roller skate and a boat when part of the road was flooded, and quite a lot of smoke of various colours.
Going looking for conkers. Most kids would use a stick to knock them down. I used a grappling hook.
Boyish things. diy crossbow, diy flamethrower. diy forge melting and casting lead and aluminium. diy go-cart.
My brother was more into electronics, so was interested in high voltage projects like building a tesla coil. Fortunately lasers weren't around then.
A few things in the scouts that may be considered dangerous today. Winter hikes where our water bottles froze solid overnight and were fighting off hypo. - Before the days of having a mobile phone for backup. Exploring caves and on the mountains just as a patrol.