General synonyms:
* Relaxed
* Unworried
* Lighthearted
* Happy-go-lucky
* Easygoing
* Insouciant
* Spontaneous
* Unburdened
* Blithe
More specific synonyms:
* Free-spirited (emphasizes a sense of freedom and independence)
* Unfettered (emphasizes being free from constraints)
* Serene (emphasizes a sense of calmness and peace)
* Jovial (emphasizes a sense of good humor and merriment)
The best synonym to use will depend on the specific situation and the nuance you want to convey.