Harry and Elsies' uncanny ability to detect whenever someone has or is about to mention The Great Big of Everything, regardless of physical presence or being asleep, never fails to entertain.
"It's a Prairie Dog Life," when Stanley tells Lionel that it was okay for him, Mimi and Marci to cross the street because they had Dennis looking out for them, and Lionel laments "I can't believe it. My little brother is getting safety tips from a goldfish."
The "baa-baa-sheep quartet" for the "The Great Big Book of Everything" song in "Sheep and a Haircut."
In "It Pays To Be a Pelican," the Griffs visit the mall and find a game in the arcade called AstroTech. Mr. Griff comments "AstroTech? Whatever happened to Pong?," but Mrs. Griff later reveals that he spent ten dollars playing it.
This little exchange from "Baby Pictures":
[Stanley runs into house and pwns an unexpecting Lionel]
Stanley: Lionel! I need pictures of me when I was a baby.
Lionel: Yeah, I bet we have about one or two—gazillion.
Harry and Elsie's exchanges have a tendency to be pretty darn funny. But this one from The Pond Couple, in this troper's opinion, takes the cake (Everything except Harry's line is slightly paraphrased)
Elsie: It may surprise you, but even Harry and I didn't get along when we first met
Stanley: You didn't? How come?
Elsie: Well, Harry's a dog and I'm a cat
Harry: You're a cat? I thought you were a chihuahua
May very well be a Shout-Out to Animaniacs. Unlikely though