Sandy being among the few who support Betty Rizzo when she thinks she's pregnant with Kenickie's baby. Keep in mind that Rizzo was the Alpha Bitch until then and had looked down on Sandy. Rizzo even goes on to Pet the Dog by thanking Sandy for her support.
Sandy's relationship with Frenchy. Sandy gets on much better with her than the other Pink Ladies, as Frenchy comforts Sandy when she's crying after she reunites with Danny and invites her to her sleepover to cheer her up. And at the end, when Sandy says she knows what she can do to impress Danny, Frenchy agrees to help her do it without even knowing what it is.
The coach shows boundless patience and encouragement as Danny fails dreadfully at basketball, football and baseball, all because Danny was trying to find a sport that fit him, which he finds with track.
It worked too, as Danny's work on the track team let him graduate on time, unlike all the other Thunderbirds.
The gang's response to the idea that they might never see each other again? Dismiss it almost instantly, and burst into the Friendship Song "We Go Together". No way their adventures are going away just because they're graduating.
Counts as Book-Ends, as at the beginning, Sandy was fretting that she may never see Danny again because she was going back to Australia. Throughout that scene she's acting as the naysayer and Danny is trying to keep her spirits up, because he doesn't want to see her unhappy. It's the first sign that he's a nice guy underneath the bad boy persona, despite not having seen him with the Thunderbirds yet.
In the 2016 special, Kenickie's change of attitude when he hears the news of Rizzo's pregnancy, even to the point of refusing to drive at Thunder Road because he doesn't want to leave his child fatherless if it all goes wrong.
In the 2016 special, the T-birds are much more kinder than in the play and the film.
The T-Birds are a lot nicer to Eugene, to the point that Danny gives his T-Bird jacket to him, letting Eugene in the group. It helps that Eugene volunteers to rev up their car engine so that it drives faster during the climatic race. The T-Birds do not toss a pie at Eugene's face, unlike in the movie, where they do. Instead, Patty compliments him on his pie throwing, and then they suggestively leave the gym to see his "rocket", delighting the T-birds.
Putzie does not call Jan fat when he asks her out to prom, instead calling her weird. He also buys sweets for her more often, and they end the show licking a snow cone together.
Sonny is a lot nicer to Marty, even showing her the gift he brought her at prom night, and comforting her at graduation when she expresses sadness at being alone. This of course earns him her love.
Sonny respecting Marty's teenage crush on radio DJ Vince Fontaine and never trying to force his affections on her when the has a chance to meet him. Even when things don't work out with Fontaine, Sonny is the first to comfort her.