I use photoshop a fair amount and wouldn’t like to be asked to teach either the basics or more advanced uses, you are much braver, or more likely knowledgeable, than me.
One thing I would like to see instruction on is the selective use of the dodge and burn tools, as along with levels on adjustment layers they are the best ways of bringing an otherwise ordinary (flat) looking image to life whether its in colour or b&w, particularly with landscapes. With landscapes how to replace flat featureless skys with something a little more interesting can also be an invaluable lesson
Everyone will have a different opinion as to what is most useful to them but surely the best way is to use a tried and tested instruction book and work from that as a guide to the structure, content and order of various lessons even if you are not directly quoting from it.
Derek