For a door knob cat toy, take a heavy string and tie a number of odd items from around the house. I have a small section of plastic chain, a piece of ribbon tied to the string with a knot, and something shiny. On the bottom, I tied a 2 inch round bell so all this will jingle. Tie it to the door knob and you'll instantly have long lasting fun for old and young cats. It seems to never grow old like most toys and if someone tears it up it is easy to replace.
Buster only likes the bell by itself tied to a string and left on the floor. He's now 1 and 1/2 years old and still likes his toy.