The Trio:
Befriend the trio and as they tell you their secrets, discreetly write them down and publish them on the Daily Prophet (Rita Skeeter pays big Galleons for gossip like that!)
Take away their Red Vines and hide the Horcruxes they found and replace them with frauds. They'll only think they destroyed all 7 and they'll be eating Twizzlers! MWAHAHAHAHA! LONG LIVE THE DARK LORD...AND STARKID PRODUCTIONS! (Product placement intended)
Hermione Granger:
Forge a report card and put a "T" for Troll next to all her classes (Especially Charms, because there's no way you can fail that class)
On her library books (that you know very well no one but her would read), mark them "Hermione Jean Granger was here". Madam Pince'll be so pissed...
Spike Hermione's meal with a Skiving Snackbox, which'll cause her to miss class, and consequently mess up her perfect attendance record
Kidnap Crookshanks and hold him hostage until she hands over her Muggle treasures (I heard she has something called a "toothbrush")
Slip Sleeping Drought into her evening pumpkin juice while she's trying to study for her 7 N.E.W.T.'s