The ending of "Doug Gets His Ears Lowered", where Roger tries to chase off some squirrels who are laughing at the silly haircut "Fluke" gave him.
Doug trying to seduce his teacher as Smash Adams. "I was just noticing what... large, hair you have!"
Also from that episode, Doug's premature confession which lead to Mrs. Wingo saying:
Mrs. Wingo: You wrote a paper titled My First Pair Of High Heals?"
Nearly everything we've seen of Judy's art school, Moody School. In some of its art labs we've seen a guy chopping a car in half with a guillotine, a musician playing a watermelon slice like a violin, two opera singers singing in a room filled with water without oxygen gear, and a guy just floating in the air.
Doug imagining getting chased by a pot roast when he imagines what the bone he found may've been.