I'll be the old stodger voice and mention that taking 5 minutes from time to time to not be stimulated is good for mental health, and apparently creativity too.
... And then I'll put in my vote for Simon Tatham's puzzle collection. (In F-droid as Puzzles, app by Chris Boyle)
Just a nice collection of quick, simple puzzles. A perfect time killer. It's a staple that I've got on all the devices in my old 2000s PDA collection since it was also built for Windows Mobile and PalmOS.
It's a full-featured chess app, but what's nice about it is that it's also got a large amount of short chess puzzles that are available offline (unlike lichess).
I love this app. It's got like 3 dozen different games, all of which you can scale in difficulty, and works great offline. It is my go to bathroom diversion.
Anki might be worth your while if you are trying to learn something.
Load it up with a flashcard deck of something you want to remember, and it'll show you those flashcards. Lots of people use it to learn languages, but it's also good for anything that requires memorisation.
Sudoku! There's a few different apps on F-Droid, take your pick of the one that looks the best to you. The underlying game is the same, but some apps have different versions that add some complexity to keep you thinking.
pixel dungeon series is very pick up and put down-able. they're all roguelikes, f/oss, with different variations for different game experiences. right now i'm playing Experienced Pixel Dungeon because it introduces a character class I really like (the duelist: rather than being a straight fighter or a caster it's a melee weapon fighter where each type of melee weapon comes with a different special attack).
The fun in these is that for each variation the gameplay will be similar, but have different elements added. There are some that have skill trees, some that have new PC classes like I outlined above, some that are just basic nethack/rogue clones with a GUI, and a bunch that I haven't tried yet. And they're all offline, turn based, and let you save at any point so you can just pinch it off and get back to whatever you need to do in an instant.
If you like deck builders, Dawncaster is pretty fun, and saves after you finish each fight (be sure to get back to the "route" screen after beating a monster)