Welcome to Platform 9 3/4! Most of the brick wall DIYs I found used a sponge with red paint on a white sheet. I found it much easier to use a red sheet (a $4.99 flat sheet purchased at Walmart) and just paint the lines.
Dementors hung from the ceiling (a covered skeleton we have for Halloween). I replaced family photos with Harry Potter portraits I found in Google searches...plus small touches like crawling spiders were around.
The free printables for the Pumpkin Juice and the Butter Beer were found at Over the Big Moon here.
The "Dragon Eggs" were red grapes.
The Great (Dining) Hall was the most impressive to the kids – complete with floating candles! (Purchased in 2-packs at The Dollar Tree) I hung them with fine fishing line and push pins. You really don't see the holes afterwards–and don't tell your husband what you did!
The goblets were from The Dollar Tree. We stuck gems we had with glue dots. The birthday banner was made by downloading the "Magic School" font from dafont.com. The plates were clear plastic Chinet from Walmart. I used a glue stick to adhere the printed out emblems to the bottoms of the plates. (I had the gold chargers.)
If you're planning a Harry Potter party, make sure to visit my Pinterest board here.