Other wilds as already suggested Is a must play. But a strong second contender for me Is cocoon. Logic/environment puzzles, with no hand holding in any way, you have to figure out everything but the level design Is sooo good.
Honorable mention for Tunic.
I had a "portable" 286 PC with DOS. And two games. Bubbles Bubbles and a game with two chimps throwing Bananas at each other.
Seems so. Checked the sources and there's no upload/download. And testing it I can confirm the network tab in the dev tools is silent. Everything work local side. Probably with some additional work this could be converted to a PWA (Progressive Web App) and work completely offline.
I'm currently playing the Amazon version.
As it's a code for gog, you can easily use heroic launcher, it works wonderfull.
Go to desktop mode, in the discover app install heroic launcher.
Then open it, go to the store and login to gog. Now you can install any gog games and add it to steam directly from heroic launcher. Then it works perfectly out of the box in game mode.
For a game it has to be Outer Wilds...
And please do yourself a favor and play it the one time you're allowed in this lifetime if you've not yet done it, as you can.