Katrina tells Donna that she signed up for a ballet class since it was something she was into when she was younger. Amanda Schull, who plays Katrina, use to be a professional ballet dancer.
Katrina asks Mike if Rachel is doing well. Mike responds "If I told you how well, you wouldn't believe me." That's a sly nod to Meghan Markle's marriage to Prince Harry & becoming Duchess of Sussex.