teaching examples for inheritance and polymorphism

  I was asked to teach ?inheritance? and ?polymorphism? in C++.

  Usually, I would not approach teaching this way, but instead
  teach ?C++ class design? and then show what's appropriate,
  possibly teaching delegation before inheritance. But now the
  time is limited, so I cannot teach everything from ground up
  in my way, and I still want to answer those questions as asked.

  My problem is finding good teaching examples and exercises.
  Somehow, I hate teaching inheritance by ?a cow is an animal?
  or ?a rectangle is a square?, or polymorphism by
  ?cow.makeSound()? versus ?cat.makeSound()?, because those
  examples seem so artificially constructed for teaching but
  rarely arise in real-world C++ applications.

  Is anyone aware of examples where inheritance or
  polymorphism really shines in C++, that are taken from
  real-life applications of C++ or are something that can be
  obviously useful in C++ programming, but are also rather
  small and do not require too much knowledge of C++ (i.e.
  only require knowledge of classes and derivation of classes
  but not of, say, generics)? Some simple examples where
  inheritance or polymorphism really makes sense?