I had the exact same goal and ended up going with the Japanese Ocean Waves Wallpaper (White Swirl) from California Wallpaper, and it nailed that clean, minimal look I wanted. The white wave pattern adds just enough texture and movement to the wall without feeling busy or loud. It feels modern, fresh, and definitely in tune with a spa-like vibe. I used it on one wall and paired it with matte black fixtures and light gray towels – the result is super sleek and calming. It’s also removable, which is a plus if you ever want to change things up. Here’s the link to the one I used: https://california-wallpaper.com/products/japanese-ocean-waves-wallpaper-white-swi-1 – really great if you're aiming for subtle but stylish.