What are some creative, easy, DIY Halloween costumes for girls?

I am a 15 year old girl from Australia. I am in grade 11. I love reading, chocolate, fudge, Milo, pavlova and Physics. I want to work for NASA one day. My favourite books are Harry Potter and Throne of Glass, and my favourite TV shows are Doctor Who, The Goldbergs, The Librarians and Star Trek.