I have two kids and I am here to tell you that they can both play longer with a big box than they do with all the toys and video games around the house. My daughter would sleep in a box like a little 6-year-old hobo if you let her. If you really want to guarantee that your kids have more fun with a box than what is inside the box the Rolobox is for you.
This little kit is cheap at about $14 and has four wheels and a pull string. The wheels and string attach to the box and turn it into a pull toy kids can take all around. My daughter would pack that thing with Zhu Zhu Pets and these little erasers that look like different things and spend all day pulling it around the house.
I wonder what keep the wheels and pull cord from coming off easy. I would bet that there is a keeper of some sort on the back of the orange pins that hold the cord and wheels on. If not kids will get frustrated when the wheels and rope come off frequently.