General Calmness:
* Tranquil
* Serene
* Peaceful
* Placid
* Still
* Composed
* Relaxed
* Unruffled
* Collected
* Even-tempered
* Gentle
* Mild
* Soothing
* Quiet
* Reassuring
Calmness in the face of stress:
* Collected
* Composed
* Stoic
* Unfazed
* Unflappable
* Cool-headed
* Level-headed
* Steady
Calmness as a state of being:
* Peaceful
* Serene
* Tranquil
* Relaxed
* Content
* At ease
* Happy
* Joyful
Calmness in terms of environment:
* Still
* Silent
* Tranquil
* Serene
* Peaceful
* Quiet
* Untroubled
* Relaxed
Calmness in terms of personality:
* Peaceful
* Serene
* Composed
* Even-tempered
* Gentle
* Mild
* Patient
* Understanding
Figurative Calmness:
* Still as a statue
* Calm as a summer day
* Smooth as glass
* Like a gentle breeze
* Peaceful like a meadow
The best adjective to use will depend on the specific context and what you want to emphasize.