Getting a 10p mixture from the ice cream van. You used to get a decent selection of teeth-rotting goodies for that money!
Making 'bangers' from two bolts and a nut. Attach a nut to the end of a bolt and half-screw it on. Pack the thing with a length of a roll of toy gun 'caps', and screw the other bolt in reasonably tight. Chuck the thing end-down onto a hard surface to compress and set the caps off... BANG!!!
'Gutties' - aka Webley catapults. You could make an impressively large hole in the side of a corrugated-plastic greenhouse with a small stone. Not that I ever did that, mind. My rebellious neighbour did though, and I'd watch & be seriously impressed/amused!!