My sketchbook is always something I have on me, whether it's to jot down an idea for a future project, write a quote I love, or sketch people and things passing by. Often I use pen and ink, as I love the idea of the permanent mark capturing a fleeting moment. Maybe this is something I would consider a 'butterfly sketch'- something quick and with strong gesture, capturing a passing second or feeling. Occasionally I use pencil or paint in this journal, but mostly I stick to whatever's on me (usually a pen).
While below shows just a few outtakes from this sketchbook, I always have multiple journals going at once exploring other techniques and ideas.