What else can you do with an old sketchbook page?

I don't want any half-hearted sketchbooks. So today I started a project to go through my current "Reportage & Copying" book and add to some of the pages - adding tone with ink and water and pencil into line drawings. I just wanted to take an insipid page and see what I could do with it.
Looking at the results, I see that much as I love line drawing, a fully fleshed out picture with a variety of tone, sometimes created by different media, is more captivating. On the picture of the houses  I filled in with diluted ink washes, grey crayon, and yellow felt tip pen. The white window frames are done with a Tippex pen.