For that purpose a demo package has its place, IMHO, because it does not confuse the newcomer with tons of functionalities. Also it is a very good starting point for problemset-like assignments, e.g. "extend the notes package by one text field and write an upgrade script".
As soon as one understands the basic concepts it is certainly better to study a real package though. So I don't think a demo-only package for the CR would be necessary.
I'll sit down later today and ad_formize notes if nobody has done so yet.