A possible plot idea is that Percy is shrunk by a spell gone wrong. He then has to journey through the forest around Camp Half - Blood. While shrunk, he encounters a group of tiny forest spirits that are usually too shy to come out when humans are at their normal size. These spirits could either help him find a way to reverse the shrinking spell or they could be in trouble themselves and need Percy's help. He would have to use his demigod skills in new and creative ways, like using a leaf as a raft to cross a puddle or a twig as a weapon against a small monster.
One plot idea could be that Percy, shrunk to a tiny size, has to navigate the Camp Half - Blood cabins without being noticed. He might discover hidden secrets in the nooks and crannies that are normally overlooked. Another could be that he gets stuck in a magical object, like a tiny charm on Annabeth's bracelet, and has to find a way to communicate his situation to his friends. Maybe he's shrunk by a new enemy who wants to keep him out of the way during an important prophecy - related event.
Well, perhaps Percy is shrunk and ends up in the middle of a battle between two mythological creatures that he would normally be able to fight against at his normal size. Now, he has to use his wits and small - size advantages, like hiding in small spaces or climbing on the creatures unnoticed, to either stop the fight or help one side. Also, he could be shrunk and find himself in a miniaturized version of the Underworld, where he has to face tiny versions of the usual Underworld threats and find a way to return to his normal size and the real Underworld to complete an important mission.