I love the art AuntThelma posted, too. It gives your dark walls a nod, while also bringing a splash of color. It brings a restful feel to the space above the resting bench and reflects the scene out the window. Your tall furniture piece is lovely, but it could only go in the space between the tub and the pony wall, right? If you need it for storage, then you should fit it in, but I like the feel of the room without it. I happen to have a few teal things saved. Though their ocean theme does not go with the picture, they might be useful if you did use the tall furniture piece. This octopus just makes me smile!

Install LEDs in the ceiling fixture, under the hanging cabinet and above the other one to add some life to the space. Get a fun, colorful poster or art to hang on the wall at the end. Pick up the counter color in your art and any accessories you add like hand towels, etc.

I think some wood shelves or a cabinet above the toilet would be nice. It would help fill some of the blank space and provide storage. (The thought of toothbrushes being kept behind a toilet in an open basket is skeeving me out a bit). You could add some decorative elements above which should add more style to the space.

For the powder room: You could do a skirt for the sink and have storage bins behind it … Baskets or shelves above toilet: I think wallpaper would look great: