I do most of my sketchbook drawings on impulse. Over time those drawings have grown into several collections.
My current materials for sketching are inktense pencils, similar to watercolour pencils but permanent when dry -primary red, blue and yellow plus orange, green, brown and pale blue: I may swap shades at times-, a permanent fine liner and a water brush. I also carry a Pentel brush pen for quick drawings when time is tight.
Actually, let me show you: I also use kitchen towel to clean my brush, a neck pouch to keep my hands free and a pencil case to keep all my stuff together.
When I started sketching I’d only use pencil. I now prefer permanent pens so I don’t waste time correcting lines: Mistakes are inevitable. I let them be and learn for next time.