Well, Harry could use a spell to make Voldemort's snake Nagini start speaking in rhymes. Imagine Voldemort trying to have a serious conversation with his snake, but all it does is spout nursery rhymes. It would be both funny and a great way to annoy Voldemort. Another idea is that Harry could use a charm to make Voldemort's dark mark itch uncontrollably. It would be a constant distraction for him, especially during important meetings or battles.
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.
Maybe Harry could use a transfiguration spell to turn all of the twins' clothes into something ridiculous, like bright pink tutus. Since they are always the ones playing pranks, it would be a great turn of events. He could do it when they are about to go to a big event at Hogwarts, like a Quidditch match or a school dance.
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.
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.
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.
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.
In some Harry Potter fanfictions, Voldemort's pet could be a large and menacing snake, like Nagini. Since snakes are often associated with Slytherin and Voldemort was a descendant of Slytherin, it makes sense for his pet to be a snake. It could also be some sort of dark magical creature, perhaps a basilisk under his control. Another possibility is a magical raven or crow, which could be used as spies for him, flying around Hogwarts and other places to gather information.
In such a fanfiction, Harry might go to Voldemort to try and reason with him. Maybe he believes there's a chance to end the war without more bloodshed. He could offer Voldemort a way out, like exile instead of death.
One possibility could be that Voldemort has a sudden change of heart. Maybe he realizes that Harry is the only one who can truly understand his loneliness and power. So instead of killing Harry, he decides to save him from a dangerous situation, like a powerful dark magic that even Voldemort fears could destroy the wizarding world if not controlled, and in the process, they form an unlikely alliance.
Maybe a plot could involve a powerful spell that binds their fates together and they have to learn to live with each other. Eventually, they might start to understand each other's motives and fall in love.
Perhaps Harry joins Voldemort out of a sense of hopelessness. After seeing so much destruction in the wizarding world, he might think that joining Voldemort is the only way to end the war quickly. However, he soon realizes that Voldemort's methods are too extreme and tries to find a way to turn things around from within the dark side.