I believe they were pretending to be on a train. They also brought out the crayons and colored the box, and turned it upside down to hide underneath it. I made Greg save it in the garage to pull out when the kids get bored.
My winter break project, which isn't entirely finished yet, was to photograph all of the boys' toys and sort them into plastic boxes labeled with pictures of the contents. This is an example of one of the pictures I took.
I did one for the boxes of puzzles as well. I bought a roll of contact paper to help me stick them to the boxes, but it didn't stick to all the boxes correctly. I had to bring out the superglue, and I ended up getting glue all over my hands in the process.
All this work to stay organized, and I might as well have just given them boxes. :) Actually, it makes me smile to see them playing so creatively with a cardboard box. They were using an empty paper towel roll as a periscope, at one point, while sitting in the box. Ahh, the joys of childhood. I'm glad the kids aren't corrupted by all of the toys that sing and buzz and move. They enjoy a good old fashioned box just as much as I remember from my own childhood.