I love the default Notes app on iOS - because it synchronizes across all my devices, it's like having a notebook always with me.
One of the ways I use the Notes app is to capture all my thoughts and ideas, in whatever form they might happen. Every day, I create a new note and I start filling it with everything the day might bring: from a rough schedule of how I expect the day to go, to notes from books and articles I read, or podcasts I listen to, including images of stuff I find interesting, inspiring quotes, audio recordings of ideas that come to mind, and in general, anything that goes through my mind that might be useful or inspiring in the future.
Every once in a while, usually on the weekend, I'll go through my notes and pick the most interesting parts. This is how I come up with ideas for my videos / blog posts, places I want to photograph, and how I learn (some of the things I write down are good starting points to research more about the topic).