General synonyms:
* Calm
* Composed
* Easygoing
* At ease
* Unwind
* Chill
* Tranquil
* Serene
* Laid-back
* Comfortable
* Unstressed
* At peace
Synonyms for "relaxed" in a physical sense:
* Loose
* Limber
* Slack
* Slouchy
Synonyms for "relaxed" in a mental sense:
* Unconcerned
* Carefree
* Unworried
* Casual
* Informal
Synonyms for "relaxed" in a social sense:
* Friendly
* Open
* Welcoming
* Hospitable
More specific synonyms:
* Relieved: Feeling a lessening of anxiety or tension.
* Content: Feeling satisfied and happy.
* Refreshed: Feeling energized and revitalized.
* Refreshed: Feeling energized and revitalized.
* De-stressed: Having reduced stress levels.
The best synonym to use will depend on the specific context of your sentence.