Call me traditional, but I'm all about using them simply for what they were made for. But....this is pretty creative. I've heard of using them in the dryer before. And I can't imagine telling someone with a walker that they couldn't use them to make their life a little safer. So, good for the tennis ball, a jack-of-all-trades!
In fact, now that I think about it, tennis players should give this a look-they'll definitely something to do with the used, weak compression balls they surely have.
Using a tennis ball to wedge into the handle of the gas pump would be a horrifically bad idea. It wouldn't allow the pump to shut itself off when the tank was full and if it got jammed in there . . . . watch out!