A small and simple built-in shelf, from Better Homes and Gardens. You could even fit a shelf like this in between the studs in the wall, making it a pretty easy DIY project. It could work in my powder room.
When we renovated my daughters' bathroom, we tore out & replaced everything, except for the oak medicine cabinet. I removed the door & painted it the trim color and now the girls have built in shelves to hold extra hand towels and wash clothes.