As kids, my siblings and I would play for days in an appliance box that occasionally found its way to our house. A simple cardboard box turned into a house, a boat, a pit, and whatever else our imaginations could come up with until the box finally was so broken and battered that it was no longer serviceable.
The other day we bought a stroller/car seat combo for our baby, and it cracked me up when Chad immediately hopped into the vacant box. What ensued was a half hour of laughter and playfulness. I think everyone needs a big, empty box every once in a while.