Who is singing to her? Does she know him? Where did they meet? How does she react to the serenade?
Are there people around as he’s singing to her or are they alone?
What type of song is it? For example, maybe she broke his heart and he’s embarrassing her by singing an “I hate you” type of song in public and shaming her.
Have fun with this and get creative. It doesn’t have to be the typical serenade scene, though it certainly can be.
Happy Sunday and Happy Writing!