A double frosted glass shower design, with an item frame to showcase the hot and cold taps.
(Submitted by _theminerdude_)

A nice bathroom area with a toilet, using an upside-down quartz block for the toilet and a player head that is designed to look like a toilet paper roll. I added a rug by placing signs in the floor and carpet onto of the signs to create a detailed look.
(Submitted by sagemcbride)

A modern toilet that looks best using quartz stairs and Slabs with a nice trapdoor.

(Submitted by UltraWarrrior7651)

Sleek Toilet
A Toilet design that utilises the Upside Down Stair block.
Stairs are used to create this bathtub. Use A full block at the end to place your faucet.

A Minecraft Shower design. All built on a slab floor. Using Glass Panes and a lever activated water fall. Also in picture are some Towel Racks (Ladders).

Toilet and Washbasin.

See this bathroom tutorial in the video below:

The design below utilises the Cauldron for the bowl, and a slab floor. 
Don't forget to fill it with water!

The classic toilet design for those with slab floors. With optional rope flush.

Or in creative mode - you can make this look even better with lowered snow layers. 
(Submitted by TheUnknownKnower)