A minor god who feels overlooked by the Olympians might be the main antagonist. He sees the fitness challenge as an opportunity to gain power and respect. So he uses his powers to create illusions and difficulties during the challenge. For example, he might make the training areas seem like dangerous mazes or create false opponents to tire out the real participants.
It could be a rogue demigod who is jealous of Percy's popularity. This demigod decides to disrupt the fitness challenge by setting up dangerous traps and obstacles for the participants. He wants to prove that he is better than Percy and will stop at nothing to make the others fail.
A possible plot is that the fitness challenge is actually a ruse by an evil god. Percy discovers this and has to not only keep up the pretense of the challenge but also secretly figure out how to stop the god's evil plan. He gets the other demigods involved in the fitness challenge to join him in this secret mission. They use their fitness training to become stronger and more coordinated for the battles they will face against the god's minions.
One interesting plot could be that the fitness challenge is actually a cover for a secret mission. The gods have hidden an important artifact somewhere on the school grounds, and the only way to find it is by winning various fitness tasks. Percy and his friends suspect something is up when they notice strange magical auras around certain areas of the challenge. So they not only have to compete but also figure out the mystery behind the tasks. Along the way, they face challenges from other demigods who also want the artifact for their own reasons.
The Percy Jackson School Fitness Challenge was on. Percy was initially reluctant, thinking it was just a normal human thing. But Annabeth convinced him to give it his all. So, he started with the long jump. With a small leap, he soared through the air much farther than any mortal could. Next was the rope climb. He scaled it in seconds, his hands gripping the rope tightly. During the relay race, he passed the baton to his friend Grover, who used his satyr speed to keep their team in the lead. In the end, their team won, and Percy learned that sometimes, mortal challenges could be fun too.
It could be a power - hungry Atlantean noble who wants to rule not just Atlantis but also the surface world. He sees Percy as a threat to his plans and tries to stop him at every turn.
A new and powerful demon that has emerged from the darkest corners of the underworld. This demon is bent on using Percy's connection to the overworld to break through the barriers between the two worlds and unleash chaos. It has powers that are different from what Percy has faced before, like the ability to manipulate the very essence of the underworld's environment.
A powerful Amazonian sorceress who has turned against her own kind and is now trying to use dark magic to take over the world. She could see Percy as an obstacle in her path and constantly try to defeat him.
Maybe it could be a demigod who has become obsessed with power through gaming. This demigod, let's say a child of Hades, discovers a way to use dark magic and combine it with gaming technology to try and rule over all the other demigods. He sees Percy as a major threat to his plans because Percy is so popular and has strong powers, so he constantly tries to undermine Percy at every turn.
The goddess Hera might also be a great antagonist. Since she has a history of interfering in the lives of demigods, she could be jealous or threatened by Percy's rebirth. She might try to stop him from reaching his full potential in his new life or cause trouble between him and his new allies.
A powerful demigod who has been corrupted by the idea of primordial power could be the main antagonist. They might believe that by allying with a primordial or obtaining their power, they can become greater than the gods. This demigod would then oppose Percy, who stands for the protection of the current order.
An ancient and forgotten god who has been awakened. This god could be angry at the modern demigods for ignoring the old ways and decides to send assassins after Percy as a warning to the rest. They might want to bring back the old order where gods were feared and worshiped more and see Percy as a symbol of the new, more independent demigods.