To write a good Percy Jackson birthday party fanfic, focus on the relationships. Show how Percy's friends care about him. They could each contribute something special to the party based on their skills. Annabeth might use her wisdom to plan an elaborate puzzle - themed part of the party. Grover could bring some delicious wild plants for the food. And don't forget about the pacing. Keep the story moving but also allow for some emotional moments, like when Percy gives a heartfelt speech thanking his friends.
First, know the characters well. Make sure Percy and his friends act in character. For example, Percy should be brave but also have his moments of humor. Then, create an interesting setting. It could be at Camp Half - Blood or in New York City. Add some unique elements like magical gifts or challenges. For instance, a gift that has a hidden prophecy. Also, use the mythology well. Incorporate Greek gods and monsters in a fun way, like having a mini - battle with a group of harpies as part of the party entertainment.
You need to make the party feel special. In a Percy Jackson fanfic, this can be done by having some sort of divine intervention. Maybe Poseidon sends a special sea - themed blessing. Or Aphrodite makes everyone look their best for the party. Build up the anticipation for the party. Start with Percy waking up and not knowing what his friends have planned. Then gradually reveal the details as the story progresses. Include some inside jokes that fans of the series will recognize, like Percy's fear of spiders and how his friends play a harmless prank related to that at the party.