Here is a non-exhaustive list of the many ways in which plastic bottles can be used as toys.
Step 1: Rubber ducky's not the one
An empty bottle's much more fun!
Pushed underwater to make bubbles, used as a boat, used to sink other boats, filled with cold water and emptied on some unfortunate sibling's head, squeezed to squirt water or simply used for good old fashion clobbering and splashing -- the fun is endless!
Plus, you never need to clean those nasty black mildew spots. Recycle, and replace it with a fresh one. Did I say one? Fill the tub!