My family moved around a lot growing up, and I have many fond memories of building elaborate contraptions out of moving boxes.
Thankfully, I don't have to move anytime soon, but cardboard boxes are still great toddler toys. I handed this one to my kids along with some markers and they turned it into a very well-decorated car. It kept them busy for a good 45 minutes, and should entertain them some tomorrow as well.
If my kids were older, I'd probably offer glue, paper, and fabric scraps for more elaborate plans.
What do your kids make with cardboard boxes?