To get the Great Hall effect, I took them out of their holders, tied thread around the bulbs and suspended them from the ceiling of my dining room with clear pushpins. I was quite happy with them and I think the Fat Lady was too.
The invitations were printed in green ink using a font I found online called Hogwarts. I tried to use similar verbage to Harry's first acceptance letter. And I tried to address each one using descriptive terminology for each kid's bedroom. If we had no idea about their bedroom we used the Cupboard under the Stairs.
When the guests arrived they had the option to don the Sorting Hat and be assigned a house. We had several stickers for each house, but the Gryffindor stickers seemed to be the most popular.
We also found some Bertie Botts Every Flavor Beans and gave them as prizes. They are always fun. The flavors seem to change over the years. I remember getting a dirt flavored one before and I heard there was earwax and vomit too, but this year the icky ones were FISH, CHEESE, ONION, PEPPER, GRAVY, and SAUSAGE. And yes, they really taste like their names.