Have you read Christina Katerina and The Box? I loved that story as a child, and I still love it! About a month ago I noticed an offer on Freecycle for a "very large box". Of course I jumped at the chance! We barely got it home, it was so big, but oh it has been worth it. :)
The kids turned it into a fort.
A few weeks later, it became a cliff, off of which they took flying leaps of great daring. When the cliff collapsed, it became an slide (sometimes for penguins - headfirst).
Once the slide fell apart, they turned it into a chasm,
and then a new kind of fort.
Like Christina Katerina's mother, sometimes I would like my space back. But really, I love watching their creativity and active play, and it is worth giving up the living room. :)