We built houses, tunnels, tents, boats/ islands, and even an airplane. Little Man loved knocking down every house I built, at one point he told me he was the big, bad wolf who was huffing and puffing to blow it down.
The Little Misters loved being able to share their new "fort" with our neighbors young kids. I do believe we used this cardboard over and over again for a couple of weeks.
So before you recycle that cardboard- re use it, build something and watch the magic happen. Even using the cardboard as giant sheets to color or paint on would be just such a mess Mondays and I would approve of.