General synonyms:
* Unwind
* Rest
* Ease up
* Take it easy
* Chill out
* Calm down
* De-stress
* Recuperate
* Relieve
Synonyms emphasizing physical relaxation:
* Slump
* Loosen up
* Stretch out
* Settle in
* Sink into
* Unburden
Synonyms emphasizing mental relaxation:
* Decompress
* Let go
* Clear your head
* Breathe easy
* Find peace
* Quiet your mind
Informal synonyms:
* Kick back
* Zone out
* Lay low
* Take a break
* Do nothing
* Chill
The best synonym to use will depend on the specific context and the nuance you want to convey.