Here are some synonyms for "relaxed":

Calm

A calm person is relaxed and not worried or upset12.

Easygoing

Someone who is easygoing has a relaxed, friendly, and informal manner125.

Laid-back

This informal term describes someone with a relaxed, casual attitude or style125.

Carefree

A carefree person is relaxed and free from worries or responsibilities12.

Mellow

Someone who is mellow is relaxed and easygoing in temperament25.

Nonchalant

A nonchalant person has a relaxed, casual manner that suggests a lack of concern125.

Composed

Someone who is composed is calm, controlled and relaxed2.

Tranquil

A tranquil person or atmosphere is peaceful and free from disturbance12.

Serene

Someone who is serene is calm, peaceful and untroubled12.

Unruffled

An unruffled person remains relaxed and calm even in difficult situations12.