I agree with DoctorDonna - theatre is the way to go. There may also be local writing schemes such as a local teen magazine that she could get involved in - if you can't find something online there may well be hard copies in the local library. There are also lots of writing websites where she could write a nove and get feedback from others chapter by chapter.
Reading-wise, if she is a voracious reader she might enjoy setting herself challenges e.g. (I am a saddo) I once challenged myself to read through the history of England chronologically from 1066 onwards through novels. It was easily do-able, enjoyable and means that as an adult I am very useful in a pub quiz.
Building her general knowledge will add richness to her understanding of what she reads so it's a good idea to encourage her to find out the background to what she is reading, e.g. Googling bits of information she doesn't understand or historical figures, places, events, bits of science etc mentioned in fiction. If she gets into the Victorian classics, it's definitely worth getting her to understand the religious and intellectual background by learning about different mythologies including the bible (Old Testament stories and the Song of Solomon for poetry), Greek myths, Norse myths etc.
Watch good live Shakespeare. You never lose that experience of your first amazing theatre performance. For teenage girls, if you can find a tragedy starring a hot male lead, it can be, erm, formative. The Roman plays are good for this (breastplate, earnest morality, bare legs, fighting). The National Theatre Live cinema streaming of plays is great if you don't live near a good theatre.
Encourage any interest in the background to books she loves - visit places, follow up enthusiasms.
It's not extra 'work' if it's what she loves. Keeping the tone intellectual and expecting her to have strong enthusiasms and opinions, never assuming that something is too old or difficult for her, and always talking about what she is interested in is what helps the most.