As a kid my family moved around a lot. Not those quick moves across town where you can borrow your friend's truck but country to country move, across oceans. Thus I spent much of my childhood waiting for my toys to come by sea freight. Consequently, I have a number of memories of playing with boxes. Making a house, an airplane, or a tank, anything seemed to be possible. I actually cringe as I remember my five year old self running around with a serrated knife, plunging it in to the cardboard to make working doors and windows.
Still it was a lot of fun and my creations stretched my imagination and took me so many places farther than my own back yard.
As a parent, card board boxes are great tools for all kinds of play. And it's so simple. Anyone can cut a hole in a box for the door and a smaller one for the window. Give your kids some markers and they can decorate the walls of the house all by themselves. Put a few smaller boxes together and you have a network of tunnels. When your kids grow tired of them (or you do) just recycle. It's simple, easy, can entertain your kids for hours and it's free.