So... I've done that May 2023 for a holiday trip.
I left with my RPi4 and few gadgets but no Internet.
There I built https://git.benetou.fr/utopiah/offline-octopus/ and my main take away is
- you can build what is missing
and more importantly the meta take away is
- you need to iterate preparations
because just like first aid you need to be actually ready when needed and knowledge change over time. You need to actually try though, test your setup and yourself genuinely otherwise it is intellectual masturbation.
Have fun!
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.