LEGO bricks
That was my childhood
It really is hard to beat the plain lego bricks as a kid. So many possibilities. I remember the daycare/latchkey had this big lego bin, but it was inexplicably 90% 6x2 black bricks, with a few red and blue ones thrown in. Those became the best things ever though once we figured out how to build Star Destroyers from Star Wars with them. I remember it being this massive challenge to see who could build the biggest one that didn't fall apart when you lifted it off the ground.