When you say calm/zen/spa, some things that immediately come to mind are: bamboo/nice wood colors, white/soft/comfortable textures, and plants and greenery

Some things you could try:

• Skinny, semi-tall bamboo shelving unit between the toilet and the wall on the left side. Could put things like incense, small potted plants, decorative glass pieces or bowls, essential oil diffuser, etc., like: https://www.amazon.com/SONGMICS-Shelving-Multifunctional-Livingroom-UBCB55Y
• Bamboo or wicker trash can to the right between the toilet and the vanity
• Above the toilet, 2-3 off-set shelves with plants or nature-related things on it (decorative pieces of bark, plants of different shapes/sizes), maybe a small mirror or a frame picture of something zen-ful
• Big, fluffy, comfortable rug/bath mat (one big one or a set of two) for in front of the vanity, which I’m assuming is across from the shower, like: https://www.amazon.com/LOCHAS-Bathroom-Chenille-Absorbent-Washable/dp/B09CPMQKJN
• Change out the photo on the wall (it’s very city/industrial, so kind of cold), with something more green and vibrant, like: https://www.amazon.com/SIGNWIN-Watercolor-Wilderness-Illustrations-Multicolor/dp/B09TSG6FQ2
• Plants and wooden/bamboo organizer trays for the counter
• Extra extra: get the glass covers on the light bulbs above the mirror frosted