Does anyone have a good example of when one would want to redefine a
method (or define a new one) for a single object?

Ie, something like

   a = [1, 2, 3]

   def a.fred
      puts "Hi; I'm Fred!"
   end

but that someone hearing about Ruby for the first time wouldn't think was
really contrived.  It would need to be fairly simple, if that's possible.