Although it’s becoming increasingly common for younger people to use the word “relaxen” (although this is typically greeted with a frown of disapproval by older people, similar to “chillen”), the proper German word for “to relax” is “entspannen”, used with the reflexive “sich”/”mich”/”dich”, etc.:

– “Ich entspanne mich am Wochenende.” – “I am relaxing on the weekend.”
– “Nach dieser Woche müssen wir uns mal so richtig entspannen.” – “After this week we’ve really got to relax.”

Additionally, the (also reflexive) “erholen” is used to express “to relax” in the sense of recovering (perhaps after a rough week at the office):

– “Ich erhole mich beim Pool.” – “I’m relaxing/recuperating at the pool.”
– “Er erholt sich von seinem Arbeitsstress.” – “He’s recovering from his work stress.”

The nouns are “Entspannung” (f.) and “Erholung” (f.).