Extremely chill (3)
(Other definitions for zen that I've seen before include "Form of Buddhism that says enlightenment comes through meditation" , "Japanese Buddhism" , "Dibdin detective; Jap. Buddhism" , "Contemplative school" , "Form of Buddhism that says meditation brings enlightenment" .)
(Other definitions for serene that I've seen before include "Totally unagitated" , "Tranquil, unruffled" , "Calm, peaceful, tranquil" , "Not agitated" , "Calm and composed" .)