So... I understand OP asked for FOSS games, so here is 1 suggestion : HyperRogue, an SDL roguelike in non Euclidian space https://www.roguetemple.com/z/hyper/

... but I mostly come to argue that in this specific case, namely short lived entertainment, FOSSness is IMHO not as important as elsewhere. I am NOT promoting closed source software here but I want to highlight the fact that unlike an OS or a DE or any software recurrently used within that setup, e.g. a video editor or a script, a game is typically not combined with other tools and is supposed (most games at least, not all!) to be used "as-is". Namely that one typically (counter example, Minecraft, with its countless FOSS alternatives) play the game, finishes it and moves on. One does not have a game being a building block or a workflow nor do they finish it and... keep on playing it (counter examples here being online games, due to competition and collaboration, user generated content).

So yes I understand the desire to want to keep a system "pure", namely rely entirely on FOSS software but I for example enjoyed Baldur's Gate 3 last year or Elden Ring at the moment despite them NOT being FOSS precisely because... they are not central to my workflow, or the way I think. Do I wish they were FOSS? Yes, absolutely! Do I wish all the content I consume, from music to books, would be DRM-free at least? Totally and that's why I typically look for this kind of content first. Yet... if there is one place where I'm more leniant than elsewhere, it'd be games.

My 2 cents, anyway HyperRogue is cool! ;)

TL;DR: FOSS games are near but arguably when it's for a short period of time, in some cases depending on your worldview, maybe non FOSS games can be acceptable.