Simple Text Editor, which is a lightweight and easy to use text editor You can configure it save on exit and open the last file.
ReadEra PDF reader.
RadioDroid is a free and open source internet radio player. Search for stations and genres around rhe world. It’s waaay better than any of the other things on the Play Store.
Shader Editor if you like programming GLSL fragment shaders.
Krita for digital painting on a tablet.
Autodesk Sketchbook for digital painting on a phone or tablet. It has a paid tier, but the free version is great on its own.
Open Camera, open source camera software. It lets you overcome the limitations of the default app. I like using it when I want a more analog look (I disabled image noise reduction in app).
Manual Camera is a good camera app if you want total control over certain thing like focus, white level, etc.
Hacker’s Keyboard. It’s got freakin arrow keys. Plus easy access to common characters needed for programming.
And last but not least
F-Droid, which is like a play store for free and open source software (FOSS). Lots of things there.