Always make of empty space and this includes the space on the back of your bathroom. You can hang towel holders down the length, giving you room to keep several towels and washcloths. Use lower racks for small children so that they can get to them easily.
towel rods on the back of the door- great idea for a space saver. Just need to make sure there's a door stop so they don't hit the wall :) was thinking of wallpaper on the back of the door behind the towel rack