Despite being an anime taking place at a school, Yumeko does not have a backpack. So if you want to have the perfect cosplay, you won't be able to bring one when cosplaying. I do recommend you to bring at least a small black purse. It does not break the cosplay, in my opinion.  If you want little props, you should bring stuff like dice and card games—this great addition to the costume. You are a compulsive gambler, after all.