I remember sitting in a college master's class for foreign language teachers- after already having taught for a few years - and the professor was talking about teaching to the National Standards and creating units that appeal to the classes. As I thought about teaching in a rural school in Missouri, that lent itself to teaching about hunting, racing and a few other stereotypes you might come across from small towns. The vocabulary would be incredibly specialized, but it would be interesting to the kids.
Perhaps I didn't realize it at the time, but you could create a unit about fishing/hunting and still have authentic texts with many other words you would be able to pre-teach. But the textbooks made by publishers are always so watered down with things that are assumed to be important vocabulary. I can't tell you how often I have used the phrase: "subir al pirámide" (to climb/scale the pyramid). Yet that was in my Spanish 1 book with vocabulary back when I took Spanish.
When I look at all of the vocabulary in the textbooks, it's overwhelming and often (at times) silly what words are deemed important. To my students, many of those words aren't important.
I'm so thankful that through TPRS, I am able to not worry about units as much or making my class very vocabulary driven. I teach them structures and we fill in gaps. Every once in awhile I might try to throw in some words that I know are useful (modes of transportation, animals, rooms of the house, weather, etc), but we let the story take us to the destination of learning the language.
In this way, my students ARE engaged, because in a story we can talk about hunting, fishing, racing, and whatever else they might have seen in a movie. This makes the language more meaningful and something attainable, while learning a bunch of grouped vocabulary terms makes it seem like something you have to work at through memorization.
What about units? In my opinion, the units should be done moreso after students have a well-rounded vocabulary and you want to treat them more like students of the language with more emphasis on grammar (much later than Spanish 1 and 2). You might be able to dabble a little in Spanish 3 and 4 with those things, but it seems as though we really have to work on the kids using the language (from hearing it correctly used in context) before we can even try to approach any "grammar lesson."