Powder rooms are typically small and are a great place to experiment with colour or wallpaper since it's not a huge investment in time or material. The room above is one I like because it has a modern, stylish vibe. Sometimes wallpaper can feel very traditional, but there are more modern and graphic options available these days. Here's another 'dotty' look that I like, paired with more traditional fixtures.
Another great looking wallpaper is Ocelot by Farrow & Ball. After using this in a bedroom last year for a client, I was definitely a fan. In the lightest shade available, it adds a subtle pattern to a room, but nothing too bold. Here it is in a traditional setting.
If I was going to do my own I'd consider this blue & white wallpaper paired with the white sconce over the sink for a fresh, stylish look.
Inspiration more modern powder room: lonny.com
Traditional room: source unknown
White sconce: schoolhouseeletric.com