Lucius Malfoy:
Alert Lucius to the fact his son is part of many AU fanfics. When he asks what they are, explain the world of slash/yaoi and the beloved shipping "Drarry".
Ask him what he's going to do with his "pimp cane". When he claims it's a walking stick, say "Sure..."
Argus Filch:
Tell Filch Harry is the one who shaved Mrs. Norris.
Luna Lovegood:
Hold a clove of garlic to her and say you're warding off her weirdness.
One day, come up to her and say nonchalantly, "Hi Luna, and by the way, you have Wrackspurts swarming all over you. It's dreadful really."
Neville Longbottom:
Send him a fake howler from his grandmother and cue it to come when he's trying to flirt with Luna.
Alastor "Mad-Eye" Moody:
Ask him if you can try some of his "pumpkin juice".
Scorpius Malfoy:
Tell him a "Yo Momma" joke. (It's funny because we all know he has two dads...)
Pansy Parkinson:
Push her off a cliff. No seriously.
House Elves:
Paint the House Elves (with the exception of Dobby and Winky) blue in their sleep and label them "Smurfs", and for Christmas, do a red and green Santa's Helpers theme
Break in and enter several Pureblood properties and provide argyle socks for all the house elves (It's for S.P.E.W.)
Barge into the House Elves Kitchen and demand diet water with a serious face
Dursleys:
Brandish a stick at them while announcing complete gibberish in a profound way, as though it were an incantation of sorts.
Threaten to blow up Dudley like Aunt Marge, unless they hand over their Muggle treasures.
Quirinus Quirell:
Tell him talking to one 's self is the first step to insanity
Ask him if his turban just sneezed
Gilderoy Lockhart:
Lock him in a room with rogue Cornish pixies- we'll see how he handles the situation.
Fleur Delacour:
Swear really badly at her and then say "Excuse my French."
Vincent Crabbe:
Lure Crabbe into a broom closet with a trail of pumpkin pasties and lock it. Hehe. (This'll stop him from demolishing the Room of Requirements with his misuse of Fiendfyre and raw stupidity.)