I feel like the key thing that sandbox games have to get right no matter what is, balance. The appeal of a sandbox game is that you want to give your player the sense of being able to "do whatever you want, however you want" in an open environment but in achieving that you have to be able to give your players enough "tools" to work with to be able to give them an idea of what they would like to do, but you don't want instruction manuals telling them that "no, you cant do that. you can only do this thing in this specific and restrictive formula" and the challenge of designing a game like this is achieving a satisfying mix of both.

Garry's Mod, of course, does a brilliant job of handling this balance while setting itself apart from other sandbox games because of it's strong focus on physics and it's smart and subtle design that tickles your brain without you even realising it.

gm_construct is such an iconic map in not just in gmod but video games as a whole, you have these big, grey and empty industrial buildings contrasting heavily with a bright green flat grassland in front of you inviting you to play and there's enough mystery in these buildings with weird abstract rooms that make 0 sense, you really feel like exploring this game more and as a result, try to break it.
The tools give you just a brief enough description of how they work, you try them out, improvise and then you fail and that honestly the overall game loop. For me a lot of the fun i get from this game i would assume is similar to how people feel when playing jazz or doing programming, you can spend like an hour trying to build a car with boxes for wheels and sinks for thruster thinking you have a great idea for some stupid contraption only for it to fall apart miserably.

But even when you fail, you're always learning. And always encouraged to go at your own pace. Garry'd mod is one of those special games where it quite literally is not just a game, it's an all purpose software tool that you can use to create, destroy, share but most importantly have fun through the game's timeless design in being a teaching tool where the developer is silently cooperating with the player to make them learn.

i made a 20 foot tall dildo and ♥♥♥♥♥♥ myself for 30 minutes. would not do again

The classic. 'Ol Reliable. The OG.

A wide open sandbox built from HL2's core, enabling a multitude of game modes.

I'm trying not to present word salad here, in my praise of this game. It's really hard to describe, in honesty.

I'm gonna do it anyway. Creative freedom. A sort of digital third place? A very open, and social online space full of cool folks. This was back in like 2019, but I'm sure there's still nice communities out there.

I recommend this game highly.