I’m finding termux + jupyter notebook with Python really, really useful for doing the math I need to do. I’ve set up a couple of different sheets for work stuff already, and in my spare time am slowly finally learning the celestial mechanics I wanted to learn as a kid. This is definitely the grandchild of the HP-71B or HP-41CX for numeric work. (And, if you need to run programs from those beasts, there are emulators in the Android store, or the excellent Free42.)
Setup’s straightforward — google “termux jupyter notebook” and follow the instructions you find.