After several classes on perspective and composition, my Sunday morning Crealde School of Art Urban Sketching students are tasked each class in creating a page of thumbnail sketches that offer a visual tour of the campus. I often do a brief set of my own as I wander the campus and offer notes for each student. In this case I just worked in back and white to demonstrate how to cut up the image into a set of large simple dark and light shapes. In one sketch I was demonstrating that the sidewalk and grass can be very much the same light value since they are both illuminated by very bright sunlight.
The challenge of the exercise is to try and make it look like you are walking along the paths and the same objects might appear small and then larger as you approach them. Students also find that the much smaller sketches can be completed faster since large washes of watercolor are not needed to cover the sketch. Much smaller puddles of wash are easy to add with the pointy tipped water brushes many students have in their kit.
The other basic lesson is that a light object will show up best if there is something dark behind it and a dark object will appear best if there is something light behind it. These thumbnails basically use just 3 values, black, white and grey. That is all that is needed to get a sketch to jump off the page.