Set in 7th year: Buy him Covergirl concealer for his scar and whisper, "Now Voldemort won't find you." Harry: Ah, the Snatchers!

Set in 5th year: Ask Harry to tell Cedric Diggory you said "Hello"

During a Quidditch match, jinx the snitch to go into Draco's palm

Send Dementors into a Quidditch match

Set after 7th year: Tell him that he could've just travelled back in time with the Time Turner and prevented Tom Riddle from being born. When looks at you dumbstruck, shrug and say, "You're welcome."

Ask Harry if the thestrals have green eyes too

Remind him Neville should've have been the Chosen One

Pay Colin Creevey to be his personal paparazzo

Transfigure Hedwig into a toad and tell Neville you found Trevor

Tell him he can just use Oculus Repairo on his eyes to avoid wearing glasses

Swap his robes for straight jackets, his Floo Powder with itching powder, and his quills for Umbridge's blood quills (Oh, he'll have a great day)

When he asks where you're going, reply "Diagonally"

Shred his Invisibility Cloak and make a collage out of it

Tell him you know this great series by J.K. Rowling

Convince Ginny to go emo

"Hermione told Ron to tell Ginny to tell Fred to tell George to tell Jordan to tell Katie to tell Cho to tell Luna to tell Chuck Norris to tell Snape to tell Draco to tell Crabbe to tell Goyle to tell Edward Cullen to tell Captain Jack Sparrow to tell Blaise to tell Pansy to tell Tom Cruise to tell the Fat Lady to tell Moaning Myrtle to tell the Bloody Baron to tell Nearly-Headless Nick to tell me to tell you that...um..actually, I can't remember."

Steal his dream journal and suggestively raise your eyebrow, asking why Voldemort's in so many of them.