Warm up for the day. Pencil with TOMBO brush pens. When we discovered TOMBO pens, we were fascinated by the brush pen, and all the different colors. We bought a bunch of pens years ago. Then we realized they are not waterproof, and if you draw with your hand touching the paper, you're going to make a big mess of your drawing.
We don't use these pens for any professional job anymore (maybe for some quick storyboards), but we still have a bunch and doing early warm up sketches is all I can think of so we don't waste them and throw them away.