Feel free to post with more suggestions. Please keep them somewhat within the realms of reasonable however. For an indicater to where those realms are please review the list below.
*Turn your best friend’s (or worst enemy’s) cat bright pink.
*Start a random duel in the middle of the fifth floor corridor because you felt like it.
*Draw on the face of someone who fell asleep in the library and hang around to see how long it takes them to figure it out.
*Throw a birthday party for a friend.
*Lock a group of Gryffindors and Slytherins in the same room and sell tickets to the show.
*Try to melt a witch by throwing water on her.
*Sneak into the common room of another house and try not to get caught while redecorating it in your house colors.
*Turn the first floor corridor into a swamp.
*Turn the Great Hall into a swamp…during a meal.
*Host a dance party in the Great Hall for no reason at all.
*Attach the clothing of your nemesis to the Whomping Willow…while they are wearing it.
*Teach Peeves how to use a blow gun.
*Set up a pick-up game of Quidditch.
*Get into a sword fight with Sir Cadogeon.
*Set up a Slytheirn on a blind date with a Gryffindor, Hufflepuff, or Ravenclaw.
*Walk up to and kiss someone’s boyfriend/girlfriend in front of them to see how they react.
*Create a Hug A (insert house/rank/category/name) Day but don’t tell those on the receiving end of the hugs about it. Do tell everyone else.
*Present cookies to various students with a big smile that doesn’t move no matter what they say/do. Never drop the smile.
*Send your best friend flowers but say they’re from their enemy. Watch as chaos ensues.
*Go on an adventure to find the Lost Grail of Hogwarts. Forget that there is no Lost Grail of Hogwarts.
*Go ice skating on the lake.
*Start an argument between two portraits in a hallway. Switch sides randomly during.
*Sit in the middle of the entrance hall with a massive crystal ball and a brightly colored turban and loudly predict the violent and untimely and bizarre deaths of everyone that walks by.
*Have a massive Transfiguration study session.
*Get stuck on a moving staircase that doesn’t want to connect to a landing so you can get off.
*Dye your hair bright orange with bright pink stripes and pretend not to know what anyone is talking about when they bring it up.
*Learn how to fly.
*Make a first year think you’re crazy by petting/grooming/talking to a thestral. Make sure said first year can’t see thestrals.
*Pretend to be a ghost and haunt the dungeons.
*Run around the Great Hall during breakfast pretending to be a unicorn/hippogriff/monkey/etc.
*Go bobsledding on the grounds.
*Serenade random people as they pass in the hallways.
*Set up an acting troupe and perform improv in the Entrance Hall.
*Have a random party just because.
*Practice the spells you learned in DADA on unsuspecting passersby, or just on each other.
*Get another student or teacher to give you some extra help/tutor you in a subject.
*Do something to get yourself put into detention.
*Go to detention.
