We have a windowless bath off the living room and wallpapered only the wall behind the sink—then painted the rest a bold navy. I love it!

You can put shelving on that wall over the toilet. I'd add a trio of artwork on the wall that faces the entry door and use a white hand towel rather than black.

Easiest: a pretty spas color like SW Sea Salt or BM Healing Aloe.

Or, wallpaper. Go as bold or simple as you want.