This topic might be a bit too advanced for 9th graders, but you can introduce it in such a way that the class might enjoy writing about it. I'm referring to the nihilism, the belief that life is essentially without purpose, that infuses the characters of Joy/Hulga and Manley Pointer. Sometimes, nihilism is used as a defense, and one can argue that Hulga's belief in nothing, caring for nothing, is her way of arming herself against attention to her perceived deformity. Manley Pointer can be viewed as a nihilistic sexual predator.
Once the class has a general understanding of nihilism, they can examine the characters for evidence of that belief system. In the context of an argument essay, for example, a possible topic can be stated thus: In "Good Country People," Joy/Hulga and Manley Pointer can both be considered nihilists, people who believe that nothing is worth believing in and that life has no real purpose. Given what happens at the end of the story, who--Joy/Hulga or Manley Pointer--is the most successful nihilist?
Another topic, perhaps better suited to 9th graders, focuses on Joy/Hulga and simply asks the class to explain why Joy changes her name to Hulga, using appropriate quotations from the text to support their views.
I have also used the following as a topic, and it yields some very interesting reactions: Flannery O'Connor's "Good Country People" has no good country people. Argue either for or against this statement, using specific examples from the text to support your argument.