I have a good quality "water saving" type detachable showerhead. It's perfect for everything except filling the kids' plastic Ikea bathtub. So I installed (with a bit of hacking) the quick-connect system used on garden hoses on the shower head, and now we have the best of both worlds: using less hot water for showers, but quick filling of the bathtub.

If any of you entrepreneuring types want to make a kit and throw it up on Kickstarter, the idea is all yours.