Here are some synonyms for tranquil:
* Calm
* Peaceful
* Serene
* Quiet
* Still
* Relaxed
* Unruffled
* Composed
Examples:
* "The tranquil lake reflected the clear blue sky."
* "She felt a sense of tranquil peace after her meditation."
* "The music created a tranquil atmosphere in the room."
Tranquil often refers to a physical environment, but it can also describe a person's emotional state.