The cardboard box the neighbors new refrigerator came in. It could be anything. A submarine, a fort, a cave, a spaceship. Me and my friend Jerry spent hours playing in the thing. It finally collapsed and his dad threw it away. Those were the days.
I seriously loved the box, especially big ones.
In SoCal, we get a huge spread of wild mustard plants covering huge areas in the spring, with the rains. These things grow to about 4' tall, and a single spindly stalk.
They then would dry out, and, before we had fire fuel abatement regulations, they'd just stay as tall golden-brown hillsides.
My grandparents lived at the bottom of a large area of hillside that formed a basin, and, that stuff would fill every inch of hillside and basin.
So, we'd haul our boxes to the top of the hills, and, climb into the box, which we cut to be like a convertible car, and, we'd blast down the hills at insane speeds. You see a similar thing at the Little League World Champ Series, where they slide down the outfield slope on pieces of cardboard.
The mustard stalks would be pushed forward and crushed down as we hit them, and became almost like a lubricated slide. We'd slide far out into the flats of the basin before we stopped.
It was this, essentially, but in washer/dryer/refrig boxes: