I prefer silence. I am fortunate to have a couple of hours/day when my spouse and kid and dog our out, and I use that to shut everything off an just enjoy the silence. I am also the most productive during those hours, and I spend my time cleaning / cooking / just sitting down and drinking my coffee while looking out through the window.
- reading
- video games
- programming side projects
Then there's the more productive options (see how bearish I am on my side projects?), like organizing (I'm trying to implement the Konmarie method) or cleaning. I like those things less, but they're still relaxing. Or I'll make something in bulk (yogurt, cheese, boiled eggs), which is also pretty relaxing (little to no mental effort).
I usually just find myself reading, especially just before bedtime.
Soaking in a nice hot bath, reading a book. I draw my bathwater just short of scalding hot, jump in the tub, and sit back and read. Sometimes non-fiction - albeit not generally highly technical material (eg, no "Python Data Analytics" or anything of that nature) - but usually fiction if I just want to "turn my brain off" and relax.
Outside of that, just listening to music. I find it very relaxing to just sit and listen to one of Chopin's nocturnes or something of that ilk.
Meditation, cooking with my SO, playing video games, reading and napping.
Unexpectedly for me another thing that is becoming relaxing as I grow older is to use my free time to stay on top of my household's chores. Just knowing I wont have the stress of having to rush them latter on because "oh fuck it's Monday morning and I don't have a clean shirt". Still far from perfect at it because procrastination run deep in me, but it's something I'm trying to slowly improve.
If I'm really stressed, coding/working on a side-project seems to work well. On normal days listen to music, DJ, an hour or two of Netflix, playing Go on KGS (or vs computer).
Read, go for a walk in the forest, draw, meditate [0], anything that's not in front of or near a source of distraction (phone, computer, &c.)
[0] for a lot of people switching off their devices and sitting down quietly for 15 min without knowing anything about meditation will already have effects.
Being outside on my own is pretty good in general. Walking or cycling are the defaults, but maybe count as exercise? I've recently started riding a motorbike and spin on some quiet-ish open roads can be pretty rewarding...
Gym membership? That's what I do, and I try to bike the ~1 mile to the gym regardless of weather.
Other than that, there are plenty of inside options for that, like yoga (plenty of yoga series on YouTube), body weight fitness (pushups and whatnot, no equipment needed, though rings and/or pull-up bar are very handy), or aerobics (I count Just Dance games in that category), depending on what you're in to.
you took excersize off the options - and I think taking a walk and tai chi fall in that, although some may say that's not the same.
I have found the best thing to help relax is dedicating 5 minutes to write - get stuff off the brain so you can relax and stop stressing it - pen and paper is fine, notepad, whatever. Write notes, jot ideas, a todo list, some mind mapping balloons.. sometimes some rhyming song lyrics to get stress off the brain and engage the heart in a healthy way.
After things are off the brain and into the paper or digital archive I find it's easier to relax in whatever situation is next.. sleep, hanging out with people, a book..
The place I relax the most is the shower. I don't feel any pressure to think anything there and can let my mind wander. Maybe a situation like that? Such a place can be hard to find for each individual.
I would say that reading+doodling with my remarkable tablet is one of the good things that help me. I also like to play Scrabble alone or play with my kid's LEGOs.