The Dining Room we put all the places on Daigon Alley and the living room is Hogwarts.
My son has lots of things we were able to use for the decor already.
We made cards for each house that described the traits that that house has. Each people was giving a house to join and a tie and ring for that house. My youngest son we dressed in a torn oversize shirt to look like Dobby. We used brick fabric to use as the 9 3/4 wall.
We mail the Hogwarts Express Train Ticket with the invites. They were purchased from https://www.etsy.com/shop/WizardDocuments?ref=l2-shopheader-name
Honeyduke candy bar wrappers - https://www.etsy.com/shop/BrensKreations?ref=l2-shopheader-name
The banner I made by using sticker I purchased from https://www.etsy.com/shop/EclecticBeginnings?ref=l2-shopheader-name
Ollivander's wands from - https://www.etsy.com/shop/HighHeeledHousewife?ref=l2-shopheader-name
Ministry of magic decal - https://www.etsy.com/listing/180886677/harry-potter-ministry-of-magic-vinyl?ref=shop_home_active_2
Wings for the keys- https://www.etsy.com/listing/180835785/paper-wings-set-of-10-pick-your-color?ref=shop_home_active_9
Honeyduke bags - https://www.etsy.com/listing/180887193/harry-potter-honey-dukes-bags?ref=shop_home_active_8
The flying keys came to me the day of. I happen to have 10 keys laying around the house so we cut the wings and hung them up.
Don't buy a lot of Butterbeer. I bought a bottle per person and people just sampled it and I have a lot left over. I would buy a few and use small cups for samples.