Word to describe an action you do to relax/de-stress
Without further information this is a guess.
I first thought of the word "to unwind", but then realised you said it wasn't common. Then, reading your example sentence I found that "cathartic" may fit. But "cathartic" or "carthasis" goes further than to relax or destress; it has an association with purging inner emotions to release tension and is a form of therapy for oneself. The word goes back to Ancient Greece, I think it was used first by Aristotle, but in regard to drama. It is also commonly used in modern psychology and medicine, hence the therapeutic association. It's also used in ordinary pop culture or layman's terms.
Some examples of the use of "catharsis" are given at Merriam-Webster Dictionary:
Acting is a means of catharsis for her.
Painting is a catharsis for me.
Merriam-Webster Dictionary
It also has this description about its origin and development:
Catharsis and cathartic both trace to the Greek word kathairein, meaning “to cleanse, purge.” Catharsis entered English as a medical term having to do with purging the body—and especially the bowels—of unwanted material. The adjective cathartic entered English with a meaning descriptive of such a physically cleansing purge. It didn’t take long for people to start using these words figuratively in reference to emotional release and spiritual cleansing.
So the word has got to do with "release" of emotions, and many people get that from doing certain activities.
If that's not the word you were thinking of let me know I'll delete the answer.
The adjectival form of relax is relaxing
Relaxing
Something that is relaxing is pleasant and helps you to relax.
I find cooking very relaxing.
We come here once a year expecting a quiet, relaxing holiday.
After a long day at work, I enjoy cooking because it's relaxing to me.
To many, exercise is relaxing.
After a long day at work, I enjoy cooking because it soothes me. To many, exercise is therapeutic.
soothe
To calm or placate (a person, for example). To ease or relieve (pain, for example).
Having or exhibiting healing powers