shattered pixel dungeon is very cute

AAAAXY

• MineTest is genuinely more fun than Minecraft, after you get past the learning curve setting up games and mods. - https://f-droid.org/packages/net.minetest.minetest/
• UnCiv is an impressively full featured Civilization clone. It looks like Civ II, but plays more like Civ 5 or 6. (Open source 4X civilization-building game) https://f-droid.org/packages/com.unciv.app/
• Simon Tatham's Puzzles (40 single-player logic games) is the best collection of logic games on any platform, anywhere. https://f-droid.org/packages/name.boyle.chris.sgtpuzzles/
• Gurgle is a delightful Wordle clone

I haven't played any of these games on mobile but I know that these well-known open-source games have at least at some point had android ports:

• Mindustry → resource extraction and processing (with conveyor belts and factories) combined with tower defense

• Minetest → voxel game (technically an engine) designed to be extremely extendable (in fact do not play the game without installing mods or games, since it's really bare-bones without it, but that is easy though since there is a builtin online content browser and joining a server makes your client automatically download the mods needed to play in that server). Often described as an alternative to Minecraft.

• Cataclysm: Dark Days Ahead →(though I wonder how this port works, since it has dwarf-fortress-like interface with most actions hidden behind a large amount of keybindings) 2d top down semi-turned-based post-apocalyptic survival game. I mentioned dwarf fortress, and there are quite a few similarities between them. It's a very intricate game, despite its interface being quite gewöhnungsbedürftig. I actually got some new cooking ideas by looking though the many crafting recipes in the crafting section. On some screenshots the game looks like it has an ASCII interface, but although the game can be played that way, there are also tilesets and they come preinstalled.

Looking through f-droid, it seems these well-known games also have an android port (though like before, I also can't testify about their quality):

• Endless sky → Space exploration game. Very similar to another open-source game, Naev. (they are both inspired by Escape Velocity). You jump to different systems, trade, transport, fight and do different missions with one major story line.

• OpenTTD → Transport tycoon game. Very fun, especially the trains. You can extend it with different maps, vehicles and behaviours by installing NewGRFs.

• Battle for Wesnoth → Turn-base hexagonal rng combat fantasy strategy game. Has different campaigns that you can play, each campaign is composed of different scenarios that together tell you a story. During combat you have to take into account the terrain your units are on, the time of day, who each is fighting, whether the units have ranged attacks, etc.

Vector Pinball

Forkyz is good if you like crosswords. It has current crosswords from popular newspapers and other sources.