There are some things that are just such "classic" things for kids to do. To me, playing in boxes is one of those things. And it truly is classic because there's nothing you have to teach them about it. It's nothing you have to prompt them to do. They just see a box and automatically want to climb in it and play!
Matt got a big box in the mail, and left it behind the couch for a few hours before it made it's way to the trashcan. In that short time frame, the kids found it, climbed in all on their own, and had a blast!