@dlowan,
Actually managed to see the egregiously bloody obvious tonight.
I have cats that love to play with skittery objects in an empty bath.
I have cats that love to play with a virtual ping pong ball on my iPad.
I have a bunch of despised ping pong balls I bought for said cats, which were rejected for general floor play.
Light bulb: put despised ping pong ball in BATH!!!!!!
Voila....happy and active cats going nuts in bath with ping pong ball!!!
Given that they put stuff in the bath themselves to play with, it remains to be seen whether they can retrieve the ball and get it back in the bath when it bounces out. I have done that for them numerous times already.....I am doubting whether they can carry a ping pong ball, though.