All of these are classic roguelikes, a genre of games which frequently aren't much to look at. The tradeoff for the looks is that they offer vast depth and complexity... and (usually) permadeath and a learning curve that's more of a cliff. I recommend watching some yt videos about any roguelike you want to learn more about, just so a fan can explain the appeal and show off all the basics.

That said:

Caves of Qud - actually one of the prettier classic roguelikes, if you can belive it. You're a traveller in a strange and unique world of vast salt deserts, jungles, and the titular caves. There is a ton of flavorful, semi-randomly generated history (especially the ever-important tales of the sultans) and cultures, so every run feels different. There is technically a main plot, but you can just ignore it and go exploring - it's a sandbox experience. The best parts, to me, are the aforementioned flavour, the tactical combat (that can get incredibly chaotic, with screen-warping effects going off every turn), the build diversity, and delving too greedily and too deeply into the caves.

Cogmind - haven't played this one, but it's on a list. You're a robot. You're building yourself from parts as you go, fighting other robots and stealing their parts.

CDDA - one of my faves, but definitely not something I'd recommend as an intro to this genre. You're a survivor in a zombie apocalypse. Go do things and don't get bitten. It's a sandbox - survive as long as you can, achieve a self-set goal. The distinguishing feature of CDDA is how realistic it tries to be - crafting is very complex, you need to track your thirst, nutrition, and sleep, you can easily get sick or get your arm broken, the zombies can track you by sight, noise, and lingering scent... My favourite part is surviving long enough to build elaborate apocalypse death mobiles, Mad Max style.