Maybe some art or even a big mirror?

Love this! What does the vanity wall look like?

A nice large plant would be nice. I've had great luck with ferns in bathrooms- a bit cliche, but they thrive in that environment.

I would do a large piece of art (or two or a triptych) by the tub, above the bench area, then maybe something like this above the toilet (not necessarily these colors):

Again, not these colors, but just for a visual:

Paint is exactly what I was going to say. I used a similar tile and painted all the walls a spa-like pale aqua. I used Silver Mist in the middle but Sea Salt would be pretty, too. SW paint.

What a difference that would make!

Tina, it is so neutral you could use any color you love. The red is pretty vibrant and not restful/spa-like necessarily, but even red could look like a very elegant finish. I also like the deep teal in one of the pics posted. A lighter teal could work, as could a peachy/salmon color.

I LOVE the color on the walls.

eta: I realized I took credit for the pic with teal, but it was someone else with a name in another language and those letters aren't on my keyboard. So I edited above and took out the "I".

You already have a great start. I also love the color of the walls and really like the art auntthelma posted as a statement piece on the big wall space to the right of the tub in the picture above. With that art, teal would be perfect for towels and accents.

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.

Haines Coral Statue, Teal · More Info

Octopus Glass Sculpture · More Info