Here’s an interesting project that uses Arduino and RFID to measure how long you spend in the shower. Instead of using a water-flow sensor (which would be overkill), Paul Asselin has made a simple RFID waving feature to tell the sensor when he starts/stops showering. I think this is a neat idea because you could probably just install the RFID on the shower door to detect when you get in the shower and out.
I chose to use an Arduino because it’s my favorite platform and I could program and debug quickly (actually within 5 to 6 hours after starting on the same day). The RFID would detect the proximity of my card (which can be handled with wet hands) and start the timer. When I’m done, I simply wave it again and tell the timer to stop. The duration is then uploade to the website’s API section via the ethernet shield. Yes, I am pretty lucky to have a powerand ethernet port not too far from my bathroom!