We can go more green with even the most simple of tasks. In my household there are a lot of people and there are times where a cup of water can easily be forgotten and left out. We used to dump the old gross water in the sinks and just refill our cups but then we decided that instead of just dumping the water why not reuse it? There's always something else that could use that water like thirsty pets and plants.
Our family poured the water in our dog's bowl for so long that he became aware that a cup of water meant that it was going straight into his bowl which made him happy and kept him hydrated and the plants got a lot healthier with the extra water. It takes so little effort and you will feel better knowing that you are saving the water from being wasted down the drain.