I doubt this is niche to most of us in this community, but it's possible very few of us consider using a terminal on our mobile devices. I highly recommend Termux (Github, F-Droid). It's really handy if you need to self-host a web-app for times you have no wifi or data connection. You can play interactive fiction on it. You can also do your writing in it. If you rely on swipe typing or auto-complete, you can just swipe left and get an input field that works with those.
To pair with my first suggestion, I also like to put forth Unexpected Keyboard (Github, F-Droid). For the times when I need to write some code on my phone, having quick access to certain symbols without having to tap a specific button, and maybe having to tap it again, is really nice. The "swipe to corner" feature takes some getting used, but for me, it's been way faster and makes writing code on my phone bearable!