With my two growing boys, we've been going through two gallons of milk a week. In my desires to be ever-green and ever-creative, I cut an empty gallon container and threw it into the bathtub with the boys. I told Christian it was a new toy and he loved it (and thanked me profusely).
A pretty obvious use of the handled half is to collect bathtoys and then let the water drain out. It is also fun to fill it up and just watch the water pour out. (BTW, that is green bath marker on the wall, not mold.)
It can also be a megaphone for a kiddo to holler to his momma through. Shh...not too loud...baby brother might be sleeping.
And of course, it can also double as a stylish hat for a bathtime artist.