That crayon box idea sounds SO cute!
Ciarasmommy, do you live in a part of the country where it might be cold on Halloween? If so, put regular tights on her instead of capris, and a little plaid or denim skirt on top. Then accessorize with a cute top, jacket, and something in her hair that will go with the punk rock theme.
Our Halloween weather is either really chilly or unseasonably hot

so I always have to plan a costume my kids can layer under, if necessary.
My 5-year-old was given a dinosaur/dragon costume, a really nice one, so he's using that.
My 11-year-old DD is going as Willy Wonka. I got a hat at Walmart for $6, and she has black pants already. All she needs is a jacket. Time to start hitting the thrift stores.
My 15-year-old DS does plan on dressing up and going out with a bunch of friends from school. But he hasn't decided on a costume yet. I told him he'd better hurry up since I'm not about to pull off a miracle on October 29!