Perhaps the girl was kidnapped because she was wearing the hair tie in a certain way that is significant to a cult or a secret society. They believe she is a chosen one or something similar. As she's held captive, she learns about their strange beliefs related to the hair tie and tries to use that knowledge to turn the situation around and free herself.
In the fanfiction, her hair tie could be enchanted. When she's kidnapped, she discovers that she can use it to send out a distress signal only visible to those she trusts. Her bun could also hold a secret. There could be a hidden tattoo on the back of her neck that is only visible when her hair is in a bun, and this tattoo is the key to a great treasure. The kidnappers don't know this yet, but they will soon find out as the story unfolds.
The story could start with the girl going about her normal day, the hair tie casually on her wrist. Then suddenly, she's grabbed and taken away. The hair tie might play a role in her escape plan. For example, she could use it to leave a trail or as a tool to pick a lock if it's long enough and strong enough.
One possible plot could be that the hair tie has some sort of magical or sentimental value. Maybe it was given to the main character by a loved one. A villain kidnaps it to try and manipulate the character into doing something. The character then has to embark on a quest to get it back, facing various obstacles along the way.
The fact that her hair is in a bun might also play a role. It could be a cultural or traditional style in the fictional world she belongs to. When she's kidnapped, her captors might be disrespectful of this style, which angers her and gives her the motivation to resist. The bun could also be used practically during her captivity. For example, she might hide small objects in it to help her escape, like a lock pick or a small note.
In a fanfiction, the girl with the hair tie on her wrist is on a solo hike. She uses the hair tie to mark the trees along the way so she can find her way back. But then she gets lost. As she tries to retrace her steps, she realizes that the hair tie is not just a simple accessory but a reminder of her own strength and she manages to find her way back to the main trail.
One possible fanfiction could be that the girl at the waterpark is a mermaid in disguise. The hair tie on her wrist is a magical charm given to her by an old sea witch. As she enjoys the waterpark, she has to be careful not to lose the hair tie, because if she does, she'll transform back into a mermaid in front of everyone. This would cause chaos and excitement among the other visitors.
There could be a fanfiction where the girl is running away from something, like a bad situation or a bully. The hair tie on her wrist was given to her by a friend who she really cares about. As she runs, she holds onto the hair tie tightly, drawing strength from it. It gives her the courage to keep running until she reaches a safe place.
The hair tie on her wrist could be a symbol of her past normalcy. Maybe it was something she put on in a hurry before being kidnapped, like a reminder of her everyday routine. It could also potentially be used as a weapon or a tool for escape if she manages to get it off her wrist.
The plot could be that the girl is on the rollercoaster for the first time. The hair tie on her wrist was given to her by her best friend who couldn't come. As the rollercoaster moves, she feels a mix of fear and excitement. And she realizes that the hair tie gives her the courage to face her fears, just like her friend always encouraged her.
A disgruntled ex - friend could be the kidnapper. Maybe there was a falling - out between the two characters, and the ex - friend wants to get back at the owner of the hair tie. They think that by taking this precious item, they can cause emotional pain.