I have placed some message boards in the walls a trapdoor for the toilet's lid and the sinks. The sinks are made by placing a message bord on the wall...
Last year, I decided to do a Minecraft hi-tech house, with some new, never seen things, so, I decided to start with a bathroom, and, of course, a toilet. This is a great thing to show to your friends. It has regular water, and, if it's really bad, disinfectant (lava), wich I only reccommend in those cases, becouse lava takes forever going out. It's really easy to make. It takes some space, but you also get a sink (water in the wall that doesn't fall). Maybe, if this idea works, in the next minecraft update there is a toilet. So, let's get started.
Step 1: The digging
First, dig a 4x3x1 hole, and then a cross like the one in the image, the sides 2 squares deep and the center 3 squares deep. This cross will be the sewer.