"Mom of a Croc:" Martin has named all the unborn crocodiles with male names... then when he notes the temperature—which is revealed to determine the gender—he immediately has to change all the names because now all the crocodiles will be born as GIRLS.
"The Blue and the Grey:" Martin's reaction to getting turned into a tree is pretty funny, even though everyone is worried about how much that's not supposed to happen.
Or Chris being a Grey Squirrel squeaking out, "Bury acorns! Bury acorns!" when he's supposed to be finding a cure for his brother's problem.
Compounded by Martin's annoyed and completely deadpan reaction to this.
Aviva and Koki finally undo the transformation - and every animal that has climbed on Martin as a tree is perched in a precarious tower on his shoulders, including the bear. The crew explains the acorn trees' importance to various animals, unfazed by Martin swaying and hopping around to keep his balance until he bumps into the beaver that previously chewed on his bark. Then he and the tower topple.
"Cheetah Racer:" While everyone else is making a vehicle that can reach cheetah speed, Chris—who's used his creature power suit to become a gazelle—is pretty much being chased by a hungry cheetah. Made funnier that he has to run with gazelle legs.
The same episode has Martin tricking Donita into taking a honey badger instead of the cheetah cub. They drive off, then you hear a car horn and Donita and Dabio screaming for the critter to get off of them, then they run away, still panicking.
In "Race for the Hippo Disk", Chris dons the Falcon Suit to save the hippo disk, shouting "Saving creature power with creature power!"... right before he smacks into a cliff and drops it. Also, Zach's perplexed "hmmm..." at finding himself in the sky when a hippo pops his inflatable boat.
"Whale Of A Squid": Bumper and his mother, both sperm whales, making the Amphisub spin around when swimming past until the boys are totally cross-eyed.
"A Huge Orange Problem": Whenever Hugo sends the boys flying through the jungle.