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.
I'll have to try and remember where I found the Windows Mobile version of it. While there's a humongous website for downloading PalmOS programs, I haven't found one for Windows Mobile yet.