Harry loves his invisibility cloak and to prank Slytherin
1. During a Quidditch match against Slytherin
Harry could use the invisibility cloak to make the Golden Snitch disappear momentarily, leaving the Slytherin Seeker astonished.
2. Harry could sneak into the Slytherin common room
and create ghostly sounds, flickering lights, and eerie shadows to give them a good scare.
3. Create the illusion of naughty house-elves in the Slytherin common room
making it seem like they are responsible for pranks and messes.
4. Harry could place a Confundus Charm on the invisibility cloak
so that when a Slytherin student tries to use it, they end up stumbling or getting into amusing situations.
5. Send anonymous notes with cryptic or humorous messages to Slytherin students
making them wonder if they have a ghostly admirer.
6. Animagus transformation can be tricky.
Harry could make it seem like some Slytherins accidentally turned into comical or less intimidating creatures.