Our living room usually has one or more empty carboard cartons lying on their sides, because the cats enjoy playing and sleeping in or on top of them. When they get bored with one, we recycle it and get a carton of a different shape. I buy groceries at a store that gives the option of getting a box rather than bags, so we have many opportunities to get new and exciting boxes.
To encourage peaceful interactions when Basement Cat is out among the other cats, I have taken to sprinkling catnip inside the largest box. It keeps the nip from getting spread all over the house.
We've started referring to that box as "the crack house."
This place has really gone downhill since Basement Cat moved in.