A quick look around my blog will clue you in to the fact that we are big fans of cardboard boxes around here. We've used them for all sorts of things.
On this particular day we took a large sheet of cardboard we'd acquired and worked as a team (Sharli included) to create an awesome play mat. I taped a couple of markers on either side of one of the kids' blocks and drew roads, then we all brainstormed on the details.
We drew a pond like the one near our home, a place for the toy animals to live in their very own zoo, parking lots for the toy cars, etc. This thing got played with until it literally fell apart and had to be smuggled out under cover of darkness because:
Our neighbors got a new washer/dryer, and as soon as I found out I begged them to save the box for us. This thing provided more hours of fun than just about anything else we've had in our home. It stayed like this in the living room for almost a week for the kids to play in. They colored nearly every square inch of it with their crayons over the course of the week which would've been worth it all by itself.