We have one of those old bath tubs whose drain stopper has long ago been lost. This bothers me. The plastic drain plug (we've tried a couple different types) doesn't stay in when the tub is filling and so I have to bend over and hold it down until the water is at least an inch high. Annoying.
Anyway, this is the "Flip-It Tub Stopper" that you permanently insert into the drain and that you can just switch open or closed. It fits most standard tubs (even our old one--nothing in our house is standard...). Plus, it was about $10 on Amazon. I am excited by small things.