What cat doesn't like to climb into a cardboard box or play with a ping pong ball? I'm sure there are some out there, but most of mine are fascinated with either, that is if the younger cats don't monopolize them.
Here's an idea for making your cat his or her own playhouse with hanging balls.
You poke holes in ping balls and then use the string to hang them inside the box. Create a door in the box (either use the open end or seal the box and cut a smaller opening in the side). Most cats won't be able to resist going inside and batting the balls.
The one problem I see with the instructions posted on eHow is that, with my cats at least, the glue will never hold. Here are the modifications I suggest.
1. Use practice golf balls instead of ping pong balls. They already have holes in them. At Walmart, the practice golf balls are cheaper than the ping pong balls.
2. Use cotton rope or clothesline, and run the rope all the way through the golf ball. Tie off one end with a really tight knot. Depending on the height of your box, you could leave a couple of inches of rope dangling and fray it.
3. Instead of gluing the string to the box, poke a hole in the box. Poke the rope through from the inside (you can tape the end temporarily to make it easier to feed) and knot that end tightly.
You'll need to use a fairly heavy duty box, because I could see a cat grabbing the ball with both paws and pulling on it with his whole weight and crumpling a lightweight box.
I think this method will hold up better than the glue method, and you won't have to punch holes in the balls.