BIG holes in the ground, yeah and tree forts. Ours never got left alone. Was there some kind of secret childhood society of fort wreckers? If so they had a big following in Redlands, Calif.
When I was about 5 I discovered these sets of vertical slots in the wall that had to be tweezer holders, so I shoved a pair of tweezers into a pair of holes to test my theory. The resulting blue flash and WHUMP! was really neat, as were all the blown fuses in the fuse box, not to mention the electric shock and burns I got on my hand. My parent's reaction must have been extreme, cuz I can't remember it at all.
We used to chuck cherry bombs into the ditches along the orange groves. If you were lucky you'd get this mini-depth charge whoosh of frogs eggs, rotten oranges, dead leaves and mud all over the kids playing boats in the muddy water there...
We made elaborate forts filled with plastic soldiers, then took these plastic syringes we found in the trash behind the cow barn at the dairy (for treating udders???), filled them with gasoline then sprayed gas through a flame all over the soldiers. Flamethrower! The soldiers melted in agony.
But the ultimate was when Mike Rose stole a box of shotgun shells from his dad's closet. We stripped them then filled an appropriate length of plugged pipe with gunpowder, stuffed in some wadding, poured in buckshot, then more wadding. There was this old work bench with a vise in the back-back yard, we clamped the loaded pipe in the vise, then turned the bench towards the incinerator. I was of course the one who dared to try to light the priming powder on the flash hole. The whooshBANG was totally satisfying as was the sight of the gaping hole in the back wall of the incinerator. What's more, the backflash had given me a real, genyouwine powder burn on my hand (gruff John Wayne/Randolph Scott voice "Don't worry ma'm, it's just a powder burn) But the ultimate rush was looking at our cannon and seeing the plug in the back was just a wooden plug and almost completely blown out the back of the pipe. In other words, if we had rammed the wadding a little tighter, the whole thing would have blown out the back of the pipe straight into my stomach. Really cool!!!
Then there were Molotov cocktails, homemade napalm, matchhead cannons and...
As daylia mentioned above, it's a wonder we survived childhood.