These are my drawing practice (I'd say "warm-up" if I continued on with something else) that I can do in the middle of the day to help myself stay sane.
These first two were drawn with the reference photo upside down--a most excellent right-brain exercise (even though the whole right/left brain thing has been scientifically debunked).
The next two were essentially blind contour drawings, with pencil, that were then rendered as realistically as possible.