mindustry

I time travel with unciv. This is how I once survived a 15 hour long haul flight.

If you like an RPG, Diablo style, there is also FLARE (Free/Libre Action Roleplaying Engine)

Lichess.org for online chess, or (most private of all) a physical travel board if you're with a friend who wants to play. Lichess isn't very private but definition though. Not sure why you asked in c/privacy? Anyway what happened to reading a book on a bus trip?

Oh try universal paperclips if you haven't. 5 hours is about right for finishing it. That's pretty private.

I usually turn to shattered pixel dungeon

Don't know what you are into, or playing on but I've been enjoying endless sky recenty:

https://endless-sky.github.io/

Doom is always great. FOSS up to Doom 3, but you still need the full games for the assets. If you don't have them there is still Freedoom. And The Dark Mod is a great free Thief like game on the Doom 3 engine.

0AD. Similar to Age of Empires

friday night funkin is surprisingly apache

Anuto TD and Breakout 71 are good time killers on the phone. Open TTD if you are into sims and Cataclysm: Dark Days Ahead if you want post apocolyptic turn based survival horror rougelike. There are also quite a few RPG fangames based on Lisa the Painful.

OpenTTD

"OpenTTD is an open source simulation game based upon Transport Tycoon Deluxe."

5 hours is enough to get addicted to this crack...

Balatro

maybe try out pico8

Install Warzone 2100, start poking the game files, adjust various numbers and see what happens. Give your cannons massive range, make every unit super cheap to manufacture or whatever. Your imagination is the limit. Play the game however you want. The game itself is good, but hacking it is just so much more fun.

If you are fine with VNs (visual novels) as games, there is Katawa Shoujo. There is a "remastered" version of it in F-Droid for android: https://f-droid.org/packages/sh.fhs.ksre/

If you are going to use a laptop (linux natively supported): https://www.katawa-shoujo.online/download

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.

FreeDoom + GZDoom + Infinite Doom

fgunz.net