Denkzettel is a simple Android app by Silvio Schurig for writing notes as text files and organizing them into categories (= subdirectories) on an Android smart phone. It can also use these files as check lists.
In contrast to most other apps of this type it does not integrate into an online service. The data is strictly kept on the phone. In combination with any sync tool (e.g. syncthing) you can still keep your data in sync with e.g. your desktop pc.
I love it.