My everyday wish list includes a shower head that not only saves water (this is common) - but has a "pause" button.
You could stop the water flow at any time (e.g., while shampooing your hair) and then restart the water flow -- at the same temperature.
Some people suggest you turn a shower on and off while you're using it, to save water. If you're fortunate to live where water supply is not an issue, that sudden blast of cold water (while the temperature readjusts) prevents people from this simply act of conservation.
It's a small thing. But it's a simple way to save water.