Song in episode 13 when Sakuta and Mai were on a late-night date In episode 13 of Rascal Does Not Dream of Bunny Girl Senpai, when Sakuta and Mai were on a late-night date towards the end of the episode, piano music is played. Here is the YouTube link.

What is that song?
It should be 青春ブタ野郎 (Seishun Buta Yarou). The BGM in the scene is rather faint but the piano melody line matches up. This is the first track on the OST release and clearly takes its name from the title of the series itself.