Another day N. and I reinforced this lesson with a long session of Mad Libs: I asked him for adjectives, nouns, and verbs, wrote wrote his words in the blanks, and read him the funny results. He found it much easier to identify parts of speech in printed text than to come up with his own words in each category. In particular, the fact that nouns can sometimes function as adjectives made it hard for him to think of adjectives other than colors, but it was good practice. I like the idea that with the magazine task and the Mad Libs, we looked at the same concept from opposite ends.
I've written this before about math, and I think it's true about "language arts" as well: you don't necessarily need an expensive curriculum to teach or learn its concepts. Language is all around you. "Language arts" is simply learning to name what you already do every day as a fluent speaker of your native tongue.