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.
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.