My favourite games to relax are the Picross puzzle games. (Switch, DS, 3DS, SNES, GB...). I just tune out and let the numbers guide me.
I believe my most relaxing game experience ever was Flower on PS3, also Journey by the same company.
Other games that come to mind, in various genre and no particular order : Unpacking (Switch, Steam, GOG), Tiny Glade (Steam), Abzu (Most systems), A Short Hike (Most Systems), Strange Horticulture (Most Systems)
Games that require puzzle solving but are immersive and still very relaxing (for me at least), like Outer Wilds (Most Systems), The Talos Principle (Most Systems), The Witness (Steam, GOG), The Return Of The Obra Dinn (Most Sytems)...
So called "Walking Simulators" : Firewalk (Most Systems), Dear Esther (Steam, GOG), The Vanishing Of Ethan Carter (Most Systems), What Remains Of Edith Finch (Most Systems), The Stanley Parable (Most Systems)...
For something really different, barely a game but very relaxing in its way, try Electroplankton on DS