Art over the bathtub. something that you love but is not so dear to your heart and overly expensive, so you won't be scared of water damage(just in case). It can be tile art btw..but these are heavier. Any concerns about earthquakes where you live? No worry about water damage though, with tile art. But it also can be oil paining, or inexpensive print.

Art, one wants to relly love, and curtain is still decor/function that can follow. So I might start with looking for art, but. Of course you can get inspired by a curtain too, and start with it...what's your style, how you'd describe it? I'm just thinking of specific, shorter kinds of curtains. What kind of privacy do you need on that window? Can the curtain be thinner/not cover the full window? I'm hesitant to recommend something specific too until I know more.

Simultaneously I'd take care of storage. You might want to "coral"/group things more, hid as much as can be hidden and the rest, to display in interesting containers/trays. You don't always have to buy them, you can find them in your house. I like to put toothbrushes and such in ceramic/clay mugs that I love for example. But if you don't want anything breakable in your bathroom, especially as kids use it too..can be plastic, metal, bone, mother of pearl, wood like teak..Sometimes they even put products in beautiful containers that can be washed after and reused. Pity to throw them out anyway.

In short, don't necessarily look for your containers in "bathroom decor" aisle, go by function. "Can I use X for Y?" Makes for more interesting space, though of course sometimes one can find adorable things in bathroom decor aisle too.

I can't see your floors, but one can always add a small rug/runner. To soften the space. They can be very useful too obviously. Just make sure they don't slid too much.

You can try and find a spot for a small plant, space permitting. These add so much.

You can decide to paint the walls. You don't have to, obviously. Your current colot seems to work well, and is a great backdrop for art. But different colors evoke different moods in us, so as an idea.

You can add hardware you like to the vanity

It's a nice bathroom, just waiting to be more personalized