On Saturday, "Fun, Fun, Fun" popped into my head, and I realized something about the rhyme scheme.
Each verse has the rhyme scheme AAAB. The first three lines all end with "now" too. Here's the first verse:
Well, she got her daddy's car and she cruised through the hamburger stand nowThat the fourth line doesn't rhyme with the first three and doesn't end in "now" could demonstrate breaking away from a set pattern (like a daily routine) and "hav[ing] fun, fun, fun." Alternatively, because the fourth line doesn't rhyme with the preceding three, there's the notion of something in the structure having been taken away, just like the T-bird in the lyrics.
Seems she forgot all about the library like she told her old man now
And with the radio blastin' goes cruisin' just as fast as she can now
And she'll have fun, fun, fun till her daddy takes the T-bird away
