One creative prank could be that Harry uses magic to make Umbridge's quills write only nonsense whenever she tries to write a decree. Another could be that he enchants her office so that every time she sits in her chair, it makes a loud and embarrassing noise, like a donkey braying. And he could also charm her teacup to refill with a really bitter-tasting potion instead of tea.
There could be a fanfiction where Hermione comes up with a brilliant prank. She uses a spell to make Umbridge's parchment write out all the unjust rules she has made instead of whatever Umbridge is trying to write. Every time Umbridge tries to write a memo or a note, the parchment just keeps repeating things like 'No groups larger than three' and 'No fun allowed' over and over again, driving her mad.
One interesting prank could be to magically change the Dark Lord's robes into something ridiculous like a frilly pink dress. It would be a shock to his dignity and his followers' perception of him.
Harry could use the Marauder's Map to always know where Voldemort is and then set up little magical traps in his path. For example, a puddle that turns into a sticky mess when Voldemort steps in it.
The Weasley twins, Fred and George, are often the main instigators. They have a reputation for their mischievous pranks in the original series, so it's natural that they would be at the forefront of pranking Umbridge in fanfiction. Their creativity and disregard for authority make them perfect for coming up with elaborate and hilarious pranks against her.
It lightens the mood. Hogwarts is under Umbridge's strict rule, and her presence is oppressive. Harry's pranks bring some humor and a sense of rebellion. Students start to feel a bit more hopeful and are more likely to resist Umbridge's silly rules.
In the 'Harry Potter Pranks Everyone' fanfiction, there could be some hilarious pranks. For example, a student might use a charm to make the Great Hall's ceiling show embarrassing memories of the teachers. Another prank could be to magically swap everyone's voices, so that when they talk, it comes out as someone else's voice. There could also be a prank where the Hogwarts Express gets diverted to a completely different location, and the students end up in a strange magical place full of odd creatures and unexpected challenges.
In some fanfictions, Harry discovers a long - forgotten magic charm that has the power to make a person's worst nightmares come true. He uses it on Umbridge, and she is so terrified by the nightmares that she runs away from Hogwarts. The charm could be something that is based on the person's deep - seated fears, and for Umbridge, it might be related to losing her power and being publicly shamed.
One great fanfiction could be 'The Prankster's Plan'. In it, Harry uses a series of clever magical tricks to prank Dumbledore. He enlists the help of the Weasley twins and their knowledge of prank - related magic. For example, they create a charm that makes Dumbledore's hat sing whenever he enters the Great Hall. It's a really fun read that shows Harry's mischievous side in a new light.
A really funny prank could be where a student's quill is jinxed to write only in limericks. So when they're trying to take notes in class, instead of normal writing, it comes out as comical rhymes. This can be a source of great embarrassment, especially if they don't notice right away. Also, pranks with vanishing ink on important parchments are quite common in fanfiction. The victim thinks they've written important information, but it all disappears, leaving them frustrated and others amused.
One creative way is by using his Patronus in a unique way. His Patronus could be made to deliver secret messages to the other students right under Umbridge's nose. Since she can't really stop a Patronus, Harry uses it as a communication tool to organize against her rules.