Ok, I remember another one, involving that same cousin.
We found a bag of saltpeter. Now, there was no internet back then, but we knew that if you mix it with some sugar, we could have homemade fireworks. We put it in a bottle cap, crimp it, ignite it and watch it fly.
We made “volcanos” using sand. It was quite good actually watching it erupt.
We also did some interesting stuff with granular carbide. I don’t remember how we got our hands on it though.
That’s how we kept ourselves entertained back then, I’m just glad nobody was hurt and we didn’t burn anything down with our antics.