I'm enjoying isle of arrows right now. Tower defense but instead of having a set map and same towers you get to draw cards and have to make do with what you get. Some cards give you pathing for the mobs, other expand the buildable area, others give you towers. Simple and fun. Game pauses between waves so you can take as long as you want to prepare.

Loop hero is really fun imo! Also solitarica

If you have a Netflix account, you can get Bloons TD6 for free on Android. It's fun and the microtransactions are completely unnecessary and hidden away. Beware it's a battery hog though

Also, if you like puzzles, look up the various variant sudoku apps from Studio Goya, particularly the one called "Cracking the Cryptic"

If you are comfortable with games that are not technically classified as freeware yet, but functionally, they probably should be... then emulation of older consoles is a great way to go too. While they are certainly not "legal", I don't think anyone playing them has ever gotten in trouble. Only the people that try to make money off of it find that the console companies are motivated enough to shut them down. Otherwise, it doesn't feel super risky to just play stuff. Just stick with games that are impossible to pay for if you want to be completely safe. There was a ton of good games on 16 and 32 bit consoles that you literally couldn't pay for now if you tried. And even as new as gamecube is getting pretty hard to possibly pay money for.

Newer stuff, I only feel ok emulating what I actually own. But as time goes on, newer and newer stuff becomes the new old stuff. A pretty wide variety of console emulators for android are in a good place now.

I do recommend a controller though if you go this route. Ideally one of the ones that also holds your phone for you. Either by making it into a switch/steamdeck kind of shape, or the ones that hold the phone above an xbox style controller. Both are good.

Grim Quest and Grim Tides. Two games by the same person. I haven't played Grim tides yet, but it's also free. Grim Quest is not a particularly large game, but I've still managed to play it over many years. I did a full run of each difficulty one by one. There is a bit to learn before it feels easy and casual, but it does get there. To be fair, I try to play it without dying. If I let myself die, it would be alot more casual even from the start. Dying is not heavily penalized on default settings, but you can alternately choose to play hardcore. Otherwise dying usually just means you gained less money for that run, and didn't make the game any harder. The game only gets harder when you successfully clear a dungeon. You can also just exit the unfinished dungeon without dying and keep most of what you have acquired so far on that run, minus a small fee for quitting the dungeon.

There are preset difficulty options, but you can also craft your own custom difficulty. There are things you could spend money on, I think it may have started out with ads. But I consider games with ads that you can pay to get rid of as basically a free demo, and getting rid of ads is the purchase price if you like the game. So they don't deter me if they don't basically break the game to add ads to it, or affect the flow of the game whether the ads are there or not.

Probably Scalak, a 3D puzzle game. It isn't foss and is paid.

Galaxy Genome, it's a space exploration game. Sort of a project to make a comprehensive 2D version of Elite: Dangerous. Or at least it started as that, I don't know if it has achieved and surpassed that by now. It was pretty far along last I played. And that was a while ago now. I kind of play games on and off for years, so that is mostly gonna be a running theme of all the games I post here. Games that you find yourself wanting to pick back up again, over and over.

royal match has no ads at all, its pretty fun just gotta ignore the microtransactions which is pretty easy to do at least compared to other games

Gubbins is a fun word game, it's a one time purchase and apparently part of the profits go to charity due to Hank Green investing in it in a creative way.

Emulation!

Sparklite, I don't know what it looks like purchases-wise, it was free with gamepass and had no in-app purchases when acquired that way. It's probably one of the newest games I'll post about. New enough that it was possible to get it with gamepass, lol. The game sort of has a modern retro zelda feel. I don't think it's a particularly long game, but it's certainly enjoyable and pretty casual. I think if it has transactions, reading other reviews it might be like a "purchase the rest of the game" style thing. So basically a free demo that you can then choose if you like the game enough to pay.

You can get a port of space cadet pinball from GitHub!

Alto's Adventure is a pretty chill game to play when killing time. https://play.google.com/store/apps/details?id=com.noodlecake.altosadventure

I also like the "connect the dots" puzzle games, like this one: https://play.google.com/store/apps/details?id=com.playvalve.connect.dots

Neither game requires microtransactions for the full experience. They might have ads, but my adblocker blocks them if they do.

Potion Permit is about $7, but pretty fun. If you got $10 to spare, Ex Astris is a jrpg that's pretty good.

Out of the Dark is alright.

I played a cool one called Pirate solitaire, very charming

Old School RuneScape potentially fits this description

Walk master

Antimatter Dimensions

2048 but a version from F-Droid: https://f-droid.org/packages/org.secuso.privacyfriendly2048/

Emulating a Nintendo DS and looking for stylus only games is my recommendation. Games like Kirby Canvas Curse, Kirby Mass Attack, and Warioware Touched are fun.

Gardenscapes is the original match 3 and build a mansion and is still my favorite. There are micro transactions to get additional power ups, but the fact that they are constantly updating with new seasonal events and you never run out of levels

Cosmic Combo, an iOS port of Johan Peitz’ PICO-8 game Cosmic Collapse, is good.

Word Cookies. I absolutely love it. You can pay for things, but it is absolutely not necessary and is basically only if you feel the need to use boosts that allow you to cheat your way past the level. I think it costs $2 to go ad-free.

It does give you regular popups asking if you want to buy coins, which I admit is annoying, but they are unnecessary to enjoy the game. There's also endless opportunities to get coins and other boosts for free. I have something like 80,000 coins and dozens of boosts and other than the $2 to go ad-free, I've never given them any money.

You do have to have certain word skills to enjoy it as much as I do. If you're the sort who can crush it at Boggle, it's the game for you.

Imagzle. There are "buy more levels" but there are plenty on the main one. It gives a picture, more or less, abstractly related to something. Art, movie, book, many random things

Star command, anomaly, world of goo, EDGE, knight of pen and paper...

These are some good ones I got from humble bundle over the years but many are quite dated so not sure if they're still around. No BS microtranactions