Another aspect of their girl talk could be about their future plans. They could be discussing what they want to do after graduating from Hogwarts. Hermione might be considering a career in the Ministry of Magic, perhaps in the Department of Mysteries, while Ginny could be thinking about becoming a professional Quidditch player. They could also talk about how they want to contribute to the wizarding world and make it a better place.
Their conversations could be about boys. Maybe they discuss their crushes like Hermione talking about Ron and Ginny sharing her thoughts on Harry. It's a common topic among teenage girls in the wizarding world.