When Captain von Trapp joins in the children singing for the baroness, and then the big group hug that follows.
All the nuns who lamented Maria's unsuitability for the convent gathered to bless her before her wedding, and smile happily as they watch her get married.
Reverend Mother assuring Maria that love of a man and a woman is holy and blessed.
The children start singing my favourite things, and Gretel asks "Why don't I feel better?". They keep singing, then they hear Maria's voice and run to her.
Maria and Liesl bonding; both when Liesl is warming to the idea of Maria as her governess, and when she asking her advice as her new mother.
At the beginning of the film, although we don't see them interacting, it's clear that Maria infuriates Sister Berthe; at any rate, the Sister certainly doesn't think Maria is suited to life in the abbey. At the end, though, she takes a huge risk in helping to sabotage the Nazis' cars -without batting an eyelid - in order to help Maria and her new family escape.
If you are more familiar with the film and Rolf's betrayal of the von Trapps, the fact that he actually protects the family in the stage musical and allows them to escape comes across as this.