General:
* Peace
* Calm
* Serenity
* Quiet
* Stillness
* Restfulness
* Tranquility
More specific:
* Inner peace: A state of mental and emotional calmness.
* Equanimity: A state of mental calmness, composure, and evenness of temper, especially in a difficult situation.
* Composure: A state of calmness and self-control, especially in a difficult situation.
* Placidity: A state of peaceful calmness.
* Repose: A state of rest or peacefulness.
* Solace: Comfort or consolation in a time of distress or sadness.
* Harmony: A state of agreement or balance.
* Contentment: A state of happiness and satisfaction.
The best synonym to use will depend on the specific context and what you want to emphasize.