I have a couple young kids that have no concept of money and how to save it by saving electricity or using less water. My son will stand in the shower until the giant water heater in our house is empty if you don't keep an eye on him. A new green gadget called the Waterpebble is a great visual indicator for when someone should get out of the shower to save on water.
The Waterpebble is a small, white device that is round with a wart on one end hiding a water sensor, a reset button one side, and three LEDs to show how good your shower is on the green scale. You put the thing near the drain and a green light means you are saving water, orange just right and red means you used too much water.
The thing can be reset at any time so if you want to feel green while using all the water you want, you can cheat. The battery inside the device is good for up to six months and can't be replaced. A single Waterpebble costs £5.31 with VAT including shipping.