The birds finding out about their shadows and being convinced that they're real. The episode has a number of funny moments, but one that stands out would be Chirp trying to feed her shadow because it "looks a little flat". Even the narrator thought it was funny!
Chirp: (drops some seeds onto her shadow) You simply must try these seeds from the forest. They are quite delicious!
Any scenes with Quack and the skunk can be pretty funny.
The pink Quack. Just the premise of it is hilarious!
Quack getting fat after gorging nuts in "Quack Goes Nuts".
In "The Tooth, The Whole Tooth, and Nothing But the Tooth", there's this scene after Beaver Boy thinks he can't be a beaver due to missing one of his buck teeth:
Beaver Boy: Being a beaver is all I know how to do.
Quack: I know! You can be a fish! Fish don't need teeth. (makes bubbling noises while wiggling his rump, occasionally lifting one of his legs up and shaking it) Hey?
Chirp: Quack! Beaver Boy is still a—
(Quack continues making bubbling sounds)
Chirp: Oh, forget it!
"Hide And Go Peep":
Quack screaming in terror, leaping up, and landing upside-down on Chirp when Peep reveals he was hiding between two rocks. Later the same thing happens when Peep comes out from his hiding place in some yellow flowers.
Later, Quack gets to the playground and tries to (unsuccessfully) bury himself in the sand, making a face print.
Quack hides behind a blue ball and quietly talks to himself:
Quack: My feet are cramped... I'm cramped! Maybe I should give up... No! A duck never gives up! I'll play this game forever! And ever! And ever! Andeverandeverandeverandeverand—shh! (whispering) ...and ever.
Quack gives away his hiding spot when a ladybug walks on his foot and tickles him, making him burst out laughing. Peep (who was nearby) jumps and lands upside-down on Chirp. Chirp even lampshades how much it's happened.
Chirp: Is this national fall-on-a-friend day?
"I lost a whole duck!"
When Chirp thinks Peep's met a talking log (actually just Quack stuck in the log), she lovingly says, "Oh, log, I love all the work you and the trees are doing for the birds!"
This, after a few failed attempts to get Quack out:
Quack: Now I'm stuck upside down... and wet.
"Peep's Night Out": When Squeak is telling Peep how dangerous night can be:
Squeak: Do you have what it takes to be a night creature? You must be brave! Fearless! Cool! Calm! (an owl hoots in the distance) WHAT WAS THAT!?!