An assignment that I had that they always seemed to enjoy was picking a stadium scoreboard (I initially forced Fenway park but loosened it up in later years) and recreating it responsively exclusively in html markup and hand coded CSS (JS only allowed to pass arrays). Had to go through all 3 major media queries.
That assignment was in week 4 out of 15. 95% of the kids came in with no CSS knowledge at that time.
If you learned CSS through the aforementioned frameworks that abstract it into a mishmash of barely recognizable selector abbreviations I totally understand why you would think that it was complex and unapproachable. But once you understand positioning and the box model selector by selector sky's the limit.
Blame bootstrap for the nature of today's HEADER ROW BLOCK BLOCK BLOCK ROW BLOCK BLOCK ROW state of web design. CSS has very little to do with that.
Edit: sorry for the run on sentences.