When Hermione slaps Harry's hand as he tries to look at the Time Turner (Prisoner of Azkaban). Why did JKR give that object such an ordinary name, unlike the "Pensieve", "Veritaserum", "Deluminator"?
In Order of the Phoenix, the sequence where Umbridge was stamping her authority over the school: using magic to separate pupils kissing, tuck their shirts in, Filch hammering the Educational Decrees into the wall, interrogating Snape, who replies in his usual deadpan manner, all to busybody music.
In the Deathly Hallows book, when Harry finds Sirius's bedroom, with the Gryffindor banners, the pictures of motorcycles, and the bikini-clad Muggle girls.
In the book of Order of the Phoenix: Madam Pince shrieking "Chocolate in the library!!! Out, out, out!!!!" and causing Harry and Ginny to be whacked round the head with their own bags.
@SirSamuelVimesBlackboardMonito I'm glad you remembered the Apparition trick, which I posted elsewhere. For those who haven't read it, here is how to do it:
At your Harry Potter party, lead the kids one at a time into the "classroom", which has a hoop on the floor at one end, and a cushion on the floor at the other. Place the child in the middle of the room, and tell them that they have to jump forwards while saying the three D's: Destination, Determination, Deliberation. Get them to practise this, so you can see how far they jump. Then tell them that when they do it for real, they have to be blindfolded, or they might lose concentration and get Splinched. Blindfold and spin them, lead them to the cushion, and tell them to jump off it, saying the three D's; and they must not peep for a few seconds. When they're allowed to look, they will find that they have Apprarated from one end of the room to the other!
The secret part is: after you blindfold them, someone sneakily moves the cushion to be next to the hoop. When you lead them to the cushion, they think you are taking them to where the cushion was when they saw it. You have to get the cushion moved back to that spot before allowing them to see, so keep telling them not to peep too early, or they'll be Splinched! The children loved this trick.