Lydia kissing BJ at the end of the night. Yeah, it was partially to prank Charles. But it was also genuine.
Lydia's bonding moments with Donny are unexpectedly sweet, especially the way Donny clearly understands the relationship between Lydia and BJ better than either of them realized.
Even if it was heartbreakingly sad, BJ pulling off the classic "Break Her Heart to Save Her" moment. He truly cares for Lydia and would rather have her hate him and never see her again than have her come to harm because of him.
The Bug Princess
BJ's proposal, from start to finish.
Charles giving his approval in a conversation with Lydia, including his acknowledgement that BJ really loves his daughter.
When she overworks herself to the point of exhaustion, BJ ends up tucking Lydia into her bed so she can get some rest.
The residents of the Neitherworld are so fond of Lydia, and so grateful to her for exerting her gentle influence over Beetlejuice, that they're petitioning to have a statue built in her honor. She finds this odd, but considerate.
Prince Vince's status as a Nice Guy(and a clever one who keeps track of proceedings on Tumblr) is proven when he finds a way to help Lydia while she's trapped in the Neitherworld and misses BJ. He adopts her as a sister so she would at least have some type of family while separated.
Both BJ and Lydia have to find a personal truth within themselves to use as a weapon against Erebos, and the ones they choose both qualify for this. For BJ, it's the secure knowledge that in spite of everything he is or has ever done, Lydia loves him. For Lydia, it's knowing that she and BJ are at their best when they're together.
Pretty much the entire last chapter is this. Regardless of his comments that he doesn't do "sweet," BJ can be pretty considerate about making sure Lydia gets what she wants. Making sure that she doesn't realize he's digging for info, he finds out all the information he needs about what she wants the wedding to be like, then throws her a surprise wedding - at the home he's just bought her.
He also shows his gratitude to Prince Vince for helping Lydia by asking him to be his best man, admitting to Lydia that the way he took care of her means that the prince is the best man he knows, so he was the logical choice.
As mentioned in the previous story, BJ asking Prince Vince to be his best man. This time, we actually get to see him ask the question.
The fact that Prince Vince is the one who urges BJ to flat out say the words: "I love you".
The entire wedding ceremony, from start to finish.
BJ making things just a little more special by using his powers to make a meteor shower that evening.