A soft (2B) graphite pencil and a blending tool are great fast sketching media. The trick is use the blender not just as a tool to smooth pencil strokes but as a drawing tool itself. Once you get graphite on the tip of the blender you can use it like a paintbrush to add value into areas of white paper. Warning: this is fun so do not overdo it- be sure not to cover all your whites with blended gray.
Click on the first image below to start a step-by-step slideshow.