Saw this here http://www.apartmenttherapy.com/how-to-make-a-wall-easel-infarrantly-creative-183699, but it was made by Becky elsewhere (link in article, I believe). As she says: "What small kid doesn't like to draw on the wall?"
12 Cool DIY Cardboard Playhouses and Toys for Kids
All of them are so cute I want to live in one. Most of them you can make from just a couple of cardboard boxes. Perfect craft and then hours and hours of fun play for kids.
Now this is a cardboard castle! Apparently they want to protect the home office from invaders (or kids).