Mini Metro, mentioned above, is seconded.
Book of Demons can customize play sessions for as long or short as you want, and is resumable if even those get interrupted.

I also second the "10tons" games recommendations (Jydge, etc).

Also, 4x games (like Civilization 4) might fit. Each game is long and sprawling, but you can play turns individually.

Despite being a great game and super relaxing, I'll counter-recommend Stardew Valley. Its days can go on (though can be short). BUT if you forget to pause, it will just keep going and going.

There is a very real danger though of "Just one more turn. Just one more turn. Just one more turn. Oh look - daylight..." :D

Maybe The Elder Scrolls IV: Oblivion (or another Bethesda game, but of the ones that are available here I'd say it is the most accessible one). It is huge and maybe not as "easy" (meaning "not complex") as Skyrim with its streamlined character system, but as character systems go in RPGs, it's quite intuitive (Skills are increased by using them, so fighting with a sword makes you better at swordfighting, etc.), and you can always just do whatever you want. Feel like exploring a dungeon? Go exploring. Feel like doing quests? Join a guild and do a few tasks for them. Just want to ride into the sunset and see what comes up? Go ahead. There is a main questline, but that's actually one of the weakest points of the game. It can be played whenever you like, or even completely ignored. And it is not a "difficult game", as in a way you cannot really fail - sure, your character can die, but you can always reload and if a fight is too tough, lower the difficulty. However, essential NPCs cannot die (in contrast to Morrowind), so you won't have to worry about permanently ruining your game. But as it is a Bethesda game, there are bugs, so make sure you have multiple save games, just in case.

The same, in different shades, also applies to Morrowind (but it is generally less accessible and may be a bit too complex with its large amounts of text and lack of comfort functions), Fallout 3 (often called "Oblivion with guns", but set in a depressing post-apocalyptic world, which may not be what you are looking for) and Skyrim (only available on Steam). Probably Fallout: New Vegas as well, but I haven't played that one, yet.

Of course, as previously mentioned, it depends on the kind of game and how you like to play them.
Those are my recomendations based on what I played, probably missed a lot...

I like fighting games for a fast gaming session.
*Skullgirls (not available on GOG)
*Street Fighter/Mortal Kombat
*Brawlhalla/Rivals of Aether (Multiplayer on Steam)

Turn based games that "can" be played on shorter sessions:
*Into the breach is a great chess-like turn based strategy game.
*Worms series - this may well be the best recomendation

Arcade/"rogue like" stuff:
*Spelunky (there is a free version on the website)
*Super crate box - awsome arcade game, ported to raspberry as well.
*the all mighty Metal Slug series - played a lot on arcade and PS1 wich is a very nice port. Metal Slug X is the best for me.
*Dead Cells
*Binding of Isaac

Bit of a cheat here as it's not on GoG (though i wish it was) Islanders. It's the perfect mix if relaxing city building and puzzle. Can play it for any amount of time and get a lovely experience from it.

As for GoG stuff -

Dead Cells - Can be tricky, but it's such a smooth play that's pretty great for pick up and play or long running sessions.

Into The Breach - Short, fun, can be deep if you want to dive into it

Project Warlock - old school themed FPS, short levels make it pretty sweet for pick up and play on short time durations


There's a ton more so i'll add few later in the day :)

I've struggled with finding the same. I enjoy RPGs, but often they take too much time investment and often require a level of commitment and attention that can't always give.

So, what did I find?

King's Bounty Warriors of the North

It's fun, light, quests are pretty simple, and combat is turn-based and tactical but not too taxing. The entire experience I find quite relaxing and easy to hop in and out when needed.

Great game, but nothing about it is "easy" :D Except dying. A lot.

However, a game usually doesn't last very long, so it's definitely possible to play for shorter periods. It can be pretty infuriating, however, and to get anywhere a certain amount of time needs to be invested.

I'll recommend West of Loathing, pretty pick up and play gameplay, it's fun and funny, can be played in short sessions, it's definitely something I'd play to chill, even though it's got some dark and horror elements (albeit done humorously)

Rayman Origins is pretty easy and each level is pretty short. It takes longer if you want to maximize your score and find all secrets, but that's not a requirement to finish the game or to have a ton of fun with it. Some of the boss fights are liiiiitle bit tricky though, but not much.

...

This.

I also bet the 2-year-old would love to watch Rayman, and later, pick up the second controller and play it coop with you. That day might come sooner than you think.

Wow! What an active community!
Thank you for all your recommendations! I only hope I get to go through all of them.

Rayman reminded me of the old Jazz Jackrabbit, looks really fun!