The song that starts playing when Lydia remembers kissing Stiles and then continues through the rest of the memories is the same song that played in the original scene for the first kiss ("Start of Time," by Gabrielle Aplin).
First reference to Isaac since his departure, when Lydia refers to the tank as "colder than that ice bath you guys put Isaac in" (as Isaac was where they learned the practice for finding lost memories that they employ here).
The corridor at the high school with the lockers is where Lydia was when she and Scott entered Stiles's mind in episode 3x22, "De-Void."