I learned about Unciv from Lemmy and have been way more addicted to it than I am to the actual Civ. I'm not certain the addiction is a good thing, but the game itself is. Highly recommended for anyone into that type of game!
Endless Sky has a mobile port on F-Droid. You fly a ship around, doing trade, missions, attacking pirates (or being a pirate). There are also a few storyline missions, many hours of fun and the game is still in development. There are some quirks on mobile but I put many hours into it, it's good fun.
Mindustry is also fun, and has many hours of potential. It's also a port of a PC game, this one works very nicely on mobile. It's sort of a Factorio (conveyor belts) tower defense game.
For a serious game I'd recommend UnCiv, imho the best adaptation of Civ to the mobile form, even counting official games. Very frequently updated.
For casual games, I like:
Lona: like Snake from old feature phones, but with circular motions. You'll understand it when you see it. Familiar and fresh gameplay at the same time. Difficult to master and keep beating your highest scores. Very addictive.
Vector Pinball: as it says on the tin, just an honest to dog, simple pinball. Many tables to choose from.
I'm also addicted to Poland can into space, which is a stupid game with bad gameplay, but fun nevertheless. :)
OpenMW - an open source implementation of the Elder Scrolls: Morrowind engine. It does require the data files from the original game though, so I guess not entirely FOSS.
That reminded me of Catan Dice Game which is a version of the roll & write of the same name. Very simple especially compared to more modern r&w games but I find the beat your own score is addictive.
Nonocross - a nonogram puzzles app with some default boards and a very simple design.
Pekka Kana 2 - an old wacky 2D platform with you commanding a rooster going from start to finish of every level.
SuperTuxKart - a classic by now, a complete parody of the original Crash Team Racing with characters being mascots from various projects and important elements of the Linux ecosystem.
(not on F-Droid) OpenRCT2 - a re implementation of the Roller Coaster Tycoon 2 engine. Free and open source, but requires the game's files.
Xeonjia a puzzle rpg where you are the newly choosen kid that must save the kingdom from the evil wizard. The main gameplay is trying to find the right path since when walking on ice you will continue to slide in that direction until you hit something. There is some light rpg elements with combat and it can be very challenging despite it's simplicity when combined with the slide mechanic and the tendency of enemies flee you.