Golf on Mars and Desert Golfing. Super simple. No ads. Its just 2D golf that keeps on going with any gimmicks.

I really like Usagi Shima. There is not much gameplay, but it's a nice game to just relax for a few minutes. Also "Really Bad Chess" is a nice twist on chess. :D

bloons td6 is a nice cozy game, one of my faves

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.

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.

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.

Walk master

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.

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.

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

Out of the Dark is alright.

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.

I played a cool one called Pirate solitaire, very charming

Old School RuneScape potentially fits this description

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

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.

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

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

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

Antimatter Dimensions

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

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

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