I stumbled across this Free illustration course over on Facebook. The course is offered by edx.org and features two experts from the University of Newcastle in Australia.

It looks like it will be both fun and beneficial to anyone seeking to improve observation and or rendering skills!
The images show a host of different subjects as well as mediums (with different approaches).
I'm not affiliated in anyway with this course of edx.com nor the University of Newcastle...I just think it looks like a super fun class! I've already signed...come and join me!