I love the way that this human computer interaction allows us to give ourselves feedback and to learn from our behavior in real time. I also love how this shower activity reminds me about the stealth shower game that Jane McGonigal wrote about in her book "Reality is Broken".
The other invention I find simply elegant is the water distiller or desalinator known as the water cone. It is easy to make, easy to maintain, and works just about anywhere. The Watercone