Cleo: "I know! It's a pizza!" (chews flat beach ball) "Yech. Definitely not a pizza..."
Pretty much the whole episode "An Itchy Patch", especially this scene:
Veteranarian: He seems to have a rash. Clifford:(Oh Crap! face)
From "Flood of Imagination":
Clifford: Okay, so if we're going to the ogre's castle to rescue Emily Elizabeth, we should take a few things with us. Like a picnic. Cleo: And some games! T-Bone: And CAKE! Clifford: And some maps. Cleo: And an inflatable swimming pool! T-Bone: And CAKE! Clifford: And gas for the car. Cleo: And 27 pogo sticks! T-Bone:And CAKE!
In "Cleo's Fair Share", Cleo imagines herself, T-Bone, and Clifford playing with her new chew toy. She imagines T-Bone getting it covered in saliva and Clifford swallowing the toy due to his huge size. The best part is how unapologetically she imagines Clifford would say "Oops."
At one point, Clifford, T-Bone, and Cleo are making funny faces at each other. T-Bone turns around so he can properly adjust his face. The result? A mildly-angry face with his tongue sticking out.
In "Jetta's Friend", Jetta's pen pal from the city is visiting and Jetta plans out the entire day and uses a timer to keep track of time. After a bird steals it, drops it, and breaks it, Jetta responds, "This was definitely not part of the plan."