I took a cardboard Uline box downstairs for recycling, and while it was patiently waiting to be taken out the kittens discovered it. And they love it. They loved the box so much that rather than take it out, I decided to convert it into a kitten gym.
I taped the top shut and then cut a circular hole for them to climb in and out of. I placed a smaller box, about 1/3 the height of the big one, under the hole so they could more easily get out. There’s also two door flaps cut in the side and two long, narrow rectangular flaps (one on the top and one of the side). The long flaps let light in, let paws out (yaknow, to attack whoever is outside the box), and the bit of cardboard flapping around gives them something to attack.
It took them all of 5 minutes to make and two minutes for the kittens to realize that this new box was the most exciting thing in the world. I consider it time well spent.