Art and Tali repairing their friendship after her Loyalty mission, especially since it turns out she was terrified for him when he went off to save her father.
Art and Tali confessing their love for one another certainly counts.
Art plays counsellor to a random asari on the Citadel. He ends up convincing her not to commit suicide, gives her enough money to go somewhere else, and starts a chain of events that saves a number of lives. He didn't have a clue who she was, he just did it because it seemed like a good idea.
Herr Wozzeck re-writing a lot of Mass Vexations 3 counts as this as well, showing that he can take constructive criticism to heart.
Art goes back to Miami and finds that his sister founded a shelter for disabled kids, in memory of his brother.