I keep a book of Mad Libs in our minivan to entertain my young daughters on car rides, and it's proven to be a somewhat useful grammar tool. I can now prompt my four-year-old for various parts of speech, and she knows that a noun is a "thing", a verb is "something you do", and an adjective "describes something". It's a shame that the resultant stories don't work perfectly, often because their creators aren't specific enough in asking for certain parts of speech. The most frequent example is when the blank calls only for a "verb", not a "transitive verb" or "intransitive verb"; most verbs are only one or the other. I'm guessing, unfortunately, that such a specification would be futile, given the general population's ignorance of grammatical terms.
My daughter's creativity makes for much more interesting choices than I'd make myself. The last time she was prompted for a noun, she gave me "A colorful butterfly with really big wings!" I guess I won't harp on the fact that that's actually a modified noun with a prepositional phrase.