One of my favorite pens that I use when I do sketches is the Micron. Many artists successfully use technical pens but I have found them to be a challenge. They seem to clog easy and it is important to clean them after each use. With the Microns they come in several nib sizes in Black or Sepia ink. Very versatile for sketching anywhere. The smallest nib .005 will give a very delicate touch and .08 will render a heavier line for a bolder look.
In the evenings I enjoy sitting down with a small sketch book and just doodling with whatever catches my fancy. Sometimes I draw from my imagination, other times from a photo. Most always though I use one of these pens to work out ideas or to write a few notes on a page.