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Season 1

Episode 1: Wake Up

The reaction Julie’s father and brother have while listening to her sing again for the first time since her mother passed away.
The message that Julie's mother left for her at the end of "Wake Up" as a final little piece of encouragement after she's gone.

When Julie explains to Flynn that she started singing again after she found her mother's song on the piano, Flynn is absolutely overjoyed that her friend is starting to look much more alive again and immediately encourages her to rejoin the music program.
It's especially heartwarming that upon the reveal that Alex is gay, Luke and Reggie don't treat him any differently and are just as supportive of him as much as any good friend would.
- The casual way it's brought up is this as well, with Luke just tossing it out in a bit of exposition. Aside from the mention of his parents "not being cool" about it, there's never any indication the others cared at all, even in the considerably less LGBT-friendly time of the 1990s.
When Reggie is feeling disappointed on how his house has been turned into a bike shack, and cannot summon things that make him happy (a puppy, a hamster and pizza), Luke begins to play "This Band is Back" to cheer him up.

After the song "Flying Solo", Julie and Flynn hug each other, repairing their friendship that was initially broken after Julie lied to her about the nature of the Luke, Reggie and Alex.

The lyrics to "I Got The Music" as Julie imagines in her head as she returns to school. It very much expresses the absolute joy that Julie is feeling now that music is back in her life, inspired by her mother, supported by her best friend and, of course, playing with the boys.

Luke notices that Alex has a crush on Willie, and is absolutely supportive for his friend and bandmate.

Both Luke and Reggie are quick to notice that Alex is feeling very anxious and frustrated about Willie, and immediately give their own piece of advice to try and cheer him up.

Julie bringing the lyrics to "Unsaid Emily" to Luke's parents, and reassuring them that, while he died young, he spent his last days doing what he loved. They are clearly deeply touched by this, having gone over twenty years likely without any sort of closure over losing their son.
We see in the flashbacks of Unsaid Emily that Bobby/Trevor was involved in the rehearsals of Unsaid Emily. Despite knowing the song and stealing all their others, he never touches that song.

Alex hugging Willie after he helps the boys despite the fact that it could result in serious consequences for him.
The passerby who hands a distressed Julie a flower outside the Orpheum, with the implication it is a sign from Rose (given that the flower is a dahlia, her favorite, and Julie had just been talking to her). Even if it's a coincidence, it's still a nice gesture, extending a bit of kindness to a crying young girl.
After being nothing but a snob to Julie all season, Carrie seems to genuinely enjoy the band's performance at the Orpheum, giving them a heartfelt standing ovation with an expression of pride for her former friend.
Carlos reveals to Julie that he knows the boys are ghosts and is choosing to willingly keep the secret from their father: "Don't ghost me", followed by a wink.
The boys pretending that they crossed over to protect Julie from more grief. And the only reason she finds out? Because they go to spend their last minutes in her garage.
Luke telling Julie that no music is worth making if they're not making it with her, and that he has no regrets about not joining Caleb's club.
Julie and Luke impulsively hug during their final scene and discover they can actually make contact. Julie immediately has Reggie and Alex come over and join in, confirming they can touch now too. When Caleb's stamps dissipate the band celebrate with a long overdue Group Hug.