Formal:
* Tranquility
* Serenity
* Composure
* Equanimity
* Placidity
Informal:
* Chill
* Relaxed
* At ease
* Unruffled
* Peaceful
* Stillness
* Harmony
* Contentment
More specific:
* Zen: A state of mental clarity and tranquility achieved through meditation.
* Bliss: A state of extreme happiness and contentment.
* Euphoria: A state of intense happiness and excitement.
* Equanimity: A state of mental calmness, composure, and evenness of temper, especially in a difficult situation.
* Stoicism: A philosophy that emphasizes virtue, reason, and acceptance of fate.
The best term to use will depend on the context and the specific nuance you want to convey.