Fleur decides that Harry should be her boyfriend. Problem is, he already has a girlfriend. However, he could do so much better (namely, her). So she comes up with an evil plan to show Harry what he is missing and why he should make his girlfriend disappear.
She invites him to a day out and they do a lot of fun activities together (eg some kind of summer or Christmas fair). She dresses up to the nines (if summer, Hotpants, crop top, you get the idea) and is very physical (hugs, playful groping and punching).
She can see that her plan is working, Harry's eyes are definitely on her, he keeps stealing glances, but no matter what, he does not take the hint. He's tempted, but he is in a relationship he has to end first. So they part ways, Fleur annoyed that her plan has failed. Only for her to get a letter where Harry invites her on a date since he has taken care of his previous relationship.
Bonus points if this breaks up Harry/Ginny and Ron is annoyed that Harry dumped his sister, only for him to reply that this is exactly what he has asked for.