Few bathrooms have enough places to hang towels. Stacking towel bars behind closed doors is a great way to remedy the shortage and use space efficiently. A flat or single-paneled door provides a crisp frame for three bars; furthermore, it's easier to affix them to a wooden door (as most are) than to a wallboard or a plaster wall. Hang the hardware according to package instructions, evenly spacing the bars along the length of the door.
Great use of under sink space. For bathroom under sink space/ use same idea using uniform Spray bottles filled and labeled/ shampoo/ conditioner/lotion/etc. keeps frequently used items out of the way/ easily accessible for use/ just unscrew sprayer.