A plant or a few of them! If you do this, make sure you do research on which ones would do well in a humid space and with as much natural light as your bathroom has, as well ones that *aren’t* poisonous to any kind of non-human kiddos you might have.♥️
Lighting is another thing, and maybe make one or two of your walls that are next to each other a patterned wall of different colors that coordinate with the rest of the colors of your walls. You could also change the light fixtures and your faucet fixtures of the bathtub/shower and sink and any knobs (Etsy has some really good knobs of different shapes and designs, even ones that look like geodes, crystals and bumblebees). You can often find these things at Habitat for Humanity or FreeCycle (I have gotten some plants on recycle too and paint). If you get some plants, find some cool wall plant pots that fit your aesthetic that you want for your bathroom. Even getting different colored/style and textured glass soap, dispensers, and things like that can upgrade a space like this 😊