I wore my Marauder's Map shirt with my time turner necklace to the event and we (my parents, younger brother, my friend and I) got to Platform 9 3/4 (the line outside the store) around 9:30 since the party started at 10, and they gave you a numbered ticket at the door so you didn't have to stand in line for the entire time.
While my parents and brother held our spot in line, my friend and I looked at the Harry Potter themed drinks in the Starbucks (The Leaky Cauldron) connected to the store to see what we wanted to order.
They had butterbeer along with themed drinks for each house. Ten points for Starbucks because Ginny was serving us as well.
I got the Slytherin Snake Bite (green tea with lemon and lime if I remember correctly), my brother got the Ravenclaw Raven (something with vanilla, coffee and blue whipped cream), and my friend got the Hufflepuff Badger Blaster (a mango smoothie-ish concoction). I wanted to go back later to try the butterbeer, but the line was huge. I did really enjoy the Slytherin though.
Also there was my city's university's Quidditch team. They had some of their brooms and one of the goals, plus a highlight reel.
There was also a dementor and a green screen that you could take pictures with, so that's what my friend and I did (as you can see).
In the middle of the store, there was a bunch of Harry Potter merchandise and colouring activities. The day before I was there and bought a Hermione Funko Pop, and I'm glad that I didn't wait until the release party because it didn't look like any were left.
At 10:30, the Harry Potter improv started. I was skeptical of it at first, but it turned out to be really great. Before it actually started they took down names and places from Harry Potter and they used these throughout the performance. The improv lasted for an hour, so there was only a half hour before the release.
About five minutes before the release, they called up the first twenty people to get in line for the release, which fortunately included my friend and me. They had all the books in boxes wrapped up with plastic (as you can see above) and the staff were not allowed to open it until precisely 12:01 (otherwise they were promised a lunch date with a Dementor).
There was a ten second countdown, and then the book was released! Once my friend and I got our books (and tote bag), we headed home where I proceeded to read the first 80 or so pages, slept, and then finished the book day of.
That about wraps it up! Did you go to a midnight release party? Have you finished the book?