The downside is that sketchbooks invite you to sketch, and I find myself occasionally doodling on the pages instead of doing my primary work.
Here's an unfinished drawing made while I was thinking about how a sense such as vision came to be possible, and what are some plausible evolutionary paths for such trait. I'm not interested just in eyes, but mainly in the processing of visual information and how knowing that could help we find new approaches for computer vision and in what tasks vision is used. Maybe that's a path that will lead us to know better what it means to see and what we can do about it.
I didn't finish it because the inking on the top figures didn't quite satisfy me, and also the monkey and rodent-like creature at the bottom weren't nice. I'll give another shot other day.
Some interesting pointers related to this subject (the evolution of vision and visual processing):