One possibility is that Ginny surprises Harry by asking him to dance first. Then, during the dance, they share a secret kiss. Another scenario could be that they find a secluded balcony at the ball. There, they can talk about their feelings for each other while looking at the beautiful Christmas decorations below.
They could have a slow dance under a starry sky charm created just for the ball. Harry would hold Ginny close and they'd look into each other's eyes, lost in the moment.
One interesting scenario could be Ginny getting ready for the Yule Ball with the help of her friends. They might be fussing over her dress, trying different hairstyles, and giving her advice on how to act around Harry at the ball.
One possible scenario could be that Ginny meets an old acquaintance who has changed a great deal and she starts to have feelings for this person while still being with Harry. Maybe they share a common interest that she feels Harry doesn't understand. However, this goes against Ginny's strong character in the original series, so it's a very different take in fanfiction.
In some versions, it could be that Harry has to die to break a dark curse placed on Ginny. He discovers that his life is the key to saving her and willingly gives it up. He might have a premonition about Ginny's fate and decides that his death is the only way to change it. He then confronts the source of the curse and allows it to take his life instead of Ginny's.
One scenario could be that Ginny has prepared a special surprise for Harry in the common room. She has decorated it with floating candles and their favorite flowers. When Harry enters, she plays a soft, romantic song on her wand - charmed radio. They then slow - dance under the candlelight, lost in their own world.
One scenario could be that Harry is really nervous when he asks Ginny. He might stutter a bit and his face could be all red. Ginny, on the other hand, is both surprised and happy. She immediately says yes with a big smile on her face.
In some fanfictions, they might go on adventures together. Exploring the wizarding world more fully, perhaps seeking out new magical creatures or ancient magic. They have a thirst for discovery after the chaos of the war. Harry's knowledge and Ginny's courage would make them a great team on these excursions.
In some fanfictions, Harry and Ginny might fight side by side, using their respective magical skills. Ginny could be shown as a fierce fighter, using her Bat - Bogey Hex to great effect against the enemies, while Harry uses his Patronus to protect them from Dementors if any are present during the battle.
They might get married and start a family right away. In fanfiction, it's common to see them settling down and having kids, living a peaceful life after all the chaos of the battle.
One possible scenario could be Ginny trying to control who Harry spends time with. For example, she might not like him being around Hermione so much because she's jealous. Another could be her trying to control Harry's actions in the wizarding world, like which quests or missions he undertakes. Maybe she thinks some are too dangerous and tries to stop him forcefully.
It's also possible that they meet at a Weasley family gathering that Harry attends. Ginny, being the youngest Weasley, is a bit shy at first but then Harry says something funny or kind, and that starts their interaction.