This is a DIY project I would love to try, we just need to buy a house first and it better have a large backyard!
I found this cardboard Cubby/ Club House while searching the net and although it dates back to 2008, I still thought it was worth sharing... This was created by Marcus Trimble from Super Collosal as part of a project for a trade show, where 6 architects where asked to create a "Sustainable Room" and this Flat-Pack Cardboard Cubby is what they came up with. I love it!

Look now you too can build your own... My mission as of today:  LOOK OUT FOR EXTRA LARGE CARDBOARD.
I'm sure we can set this up at the park, right?

Check out their plywood version too, so cool!