My favorite activity as a child was to build a fort out of a large, cardboard box.
When a large package arrived at the door, I immediately began brainstorming all the different things I could make.
I even enjoyed the smaller boxes.
I would make tons of different things with them.
Box Play for Kids, started using empty boxes and other household materials to make toys by just adding custom made stickers to them.
This is a great way to help the environment, but also, a cheap way to entertain children.