I keep sketchbooks and doddle-books. Sketchbooks are for quick sketches where freshness and directness are important. Sketches can be simple or elaborate.
Aaron Blaise is a good artists who has some YouTube videos about his sketchbooks and why he keeps them.
If you are an artist or art student, keep a sketchbook.
I also keep doddle-books. Doodle-books are sketchbooks where I let myself, well, doodle. Before I let myself doodle in sketchbooks, I doodled on any piece of paper at-hand.
I always have a sketchbook close.