I had a HP party for my son years ago.
I remember goodie bags of: small pouches that I sewed together and added a ribbon pull (if not a sew-er I think
walmart has some in the wedding supply aisle) and putting in gold coins (Natural candy store has excellent organic DARK or milk chocolate coins) And getting Jelly Bellys for the Bertie Botts (but now they have bertie botts easily available)
Did a castle cake and added HP figurines to it, pretty easy. I think many store bakeries have HP as a choice now.
For games I did HP trivia contest, you can find lots of misc facts online and make it either true/false or ABC answers so the kids don't feel dumb with no answer at all.
Maybe do a craft where they make their own wand with chopsticks and then paint, markers, glue, glitter, jewels feathers, etc
Maybe a scavenger hunt around your house or yard: I'd have a theme for each team like all the pets/owls of the series, the teachers of Hogwarts, or the Weasley family. I'd gather pics from the internet and add any words to make sure they were identifiable. Once I had the same number of objects (pictures) I would hide them around the house, yard. and team up the girls to find their theme. you could tell them exactly what they are looking for, or just the theme and how many they are looking for.
You can also make a version of quidditch with a basketball hoop and ball (maybe other kids will be tossing nerf ball while trying to make shot) Or you can use nerf ball in the house with buckets or laundry baskets.
For the games you can do prizes or to save $$ you can print out award certificates.
Oh boy I have so many ideas, I love planning parties!!!!
