I almost made the "too much time on their hands" comment but then I thought but what is it that us lot do that has our wives shaking their heads in despair.
A little over the top maybe but cats do love cardboard boxes. It's hard to keep our two out of them at times and it's surprising just how small a box they can squeeze into when the fancy takes them!
Not sure about the cat, but we have a friend who has a rabbit who would just love that, of course after a couple months the rabbit will have consumed the castle.