Seeing Max, Bobby and PJ in their dorm room like the college kids they are. The biggest deal is that it's a Disney movie - and yet they suddenly snap out on PJ and show he's slobbing in his undies while panicking about Goofy.
Max pulling off Goofy's afro wig while saying "Dad...you're scaring people" and tossing it into a trash can. And then we see Bobby wearing the same afro wig in the next scene.
The disco scene has a few:
Max making fun of Bobby when Beret Girl makes her choice, particularly the latter's face both before—an astonished wide-eyed jaw drop—and after—a peeved glare.
The simultaneous jaw drop when Sylvia arrives on the scene, followed by the DJ popping his gum all over his face.
Also, PJ and Goofy dancing, while an awesome moment for both and a heartwarming moment for at least the former, is also quite funny, particularly when Goofy grabs PJ's thumb and twirls him around and the dance moves immediately following that.
Bradley's faces, regardless of his mood, are usually hilarious, mainly because of his flat face and big teeth.