Late to the party, but since the thread got revived, maybe you'd be interested in Starbound? It's a pretty chill game, specially when playing with close friends. It's here on GOG too, and it has DRM free multiplayer, even!

There is a trick here. The game originally came with two difficulty levels, named Normal and Hard, but when they added a new harder mode they renamed the old difficulty levels:
- the old "Normal" mode was renamed to "Easy";
- the old "Hard" mode was renamed to "Normal";
- a new harder mode was added under the name "Hard".

So if you play the current "Normal" mode, it is equivalent to playing the old "Hard" mode.

If it's similar to Terraria, it might not be for me.

It's been quite a while since I last checked Terraria, but Iirc, Terraria is more focused on defeating the bosses and getting to the end game, while Starbound is more about exploration and sandboxing.

Although, perhaps worth noting that, from what I remember reading on the topic, the creator of Starbound is an ex-Terraria dev, although that's been quite a while back.

In Terraria, I really wasn't that thrilled by the gameplay itself - building stuff in 2D, however cute it may be, isn't vibing with me. "Endless" map in 2D also doesn't feel that great. I tried Terraria several times, before and after that big update or something, and it didn't click...

Some games that make me chill ;)

Ignition
Fun and sometimes funny MS-DOS racing game with nice 3D graphics.

FlatOut 2
Racing game with very nice 3D graphics, car destruction system and amazing soundtrack (rock music).

The Settlers 2
My favourite Settlers game, I simply enjoy placing roads, flags and buildings...

Torchlight 2
RPG in which player doesn't need to check walkthroughs to enjoy it. No annoying puzzles etc.

Worms Armageddon
My favourite Worms game. Funny game for whole family to play on one computer (hot-seat mode).

Check out Cloudpunk. I think it's on sale right now. It's mainly just flying your The Fifth Element style cab/car around a vibrant voxel art style 3d world, with a bit of walking around on foot. While there are some time dependent events, most of the gameplay is very chill.

Race the Sun, with the Sunrise DLC.

Townscaper.

Just remembered another one I'd recommend, and not of the sandbox type: Lyne

It's a very casual game about joining dots.

It's available DRM free elsewhere, including for Android, but dropping the community wishlist listing here just in case:
https://www.gog.com/wishlist/games/lyne

RPG chill it seems like, try Cataclysm Dark Days Ahead. Its free and lengthy. I recommend an older version "pre-pockets", which you can look to the older stable versions like version E, maybe F.

I also happen to have such a modified stable version with mods, if requested of me.

If however you like no thinking games, like a time waste. Try Hotline Miami 1 and 2. Totally bonkers time waste.

Divine Divinty in a very lengthy Diablo 2 rpg chill game too.

Stardew Valley - a game where you escape the city and build a farm, and meet the people of the small town (get married and stuff like that)

Ori and the Blind forest, platform across a beautiful land

Dorfromantik - I never played it myself but my brother says it is really chill, it is a fun hex-based city builder or something where levels have goals I think.

Seconded. They're so chill, I am unable to play these games for more than two hours. They make me drowsy and I think I actually fell asleep once.

And sometimes when I want to waste some time without a challenge I pick up one of the five entries in the Virtual Villagers series (not on Gog). They're free and extremely chill, too.

However, I think the games on Gog that closest appear to match the desired genre are Traveler's Rest or Crossroads Inn, but having played neither I have no clue whether they are chill at all.

The Tropico games are p. chill city builders on a slightly smaller scale, set on a Carribean island under your benevolent dictatorial rule.
It's realtime, but pauseable. And has you trying to make money out of such varied things as farming, mining, distilling rum and selling guns.
The main aim is to stay in power and stuff your personal bank account as much as possible, either by leaning into military dictator stuff or actually making the island a decent place to live.

Imo Tropico 4 offers the best balance between features, presentation and atmosphere.
It also has an absolutely bangin' soundtrack.


Another chill game in a completely different direction is Slipways
A space themed turn based game where the aim is to create a prosperous region of interconnected colonies within a specific time limit.
The main gimmick of the game is that colonies need stuff produced by other colonies otherwise they can't do much, and must be connected to other colonies by "lanes". Which can't cross each other.
There is an undo function so you can try various things, but it doesn't work if anything you do reveals additional information.
The whole game revolves around you making non-reversible choices with incomplete information.

Slipways Classic, made by the same dev and available elsewhere, is also pretty good, and the free PICO-8 cartridge is playable on the official PICO-8 player, and it's potentially playable on PICO-8 emulators.