Brought a cardboard box from the supermarket.
I do that once in a while as it is the perfect present for cat
besides foodCat hopped in before I even put it on the floor and now spends most of the day inside.
The box has a lid with a rectangular opening just wide enough to get in and several small holes on all sides for looking and pawing through.
While already awesome for one cat, the real fun starts when neighbour cat comes over and one is lurking in the box and the other outside. Both are old but they behave like kittens while playing, jumping in and out. It does escalate once in a while but generally they get along well.
The only thing better than a cat is moar cats.