The entire sequence after George realizes Bedford Falls is back to normal again and lets the whole town know about it. Both funny AND heartwarming due to how insane with relief and happiness George is now.
The dance scene in the gym with the pool underneath the floor.
George's mother shares this conversation with her maid during George's wedding:
Ma Bailey: First Harry, now George. Annie, we're just two old maids now.
Annie: You speak for yourself, Mrs. B.
During the final scene where Annie adds her money to the pile, she adds the line,
"I saved this money for a divorce in case I ever got a husband!"
George has a heart-to-heart talk with his father and tells him how great he is, then,
George: [raising his voice] Did ya hear that, Annie?
Annie: [behind kitchen door] I heard it; it's about time one o'ya lunkeads said it!
Every time the wooden ball on the staircase falls off. You'd think in fifteen years of marriage George would get around to fixing it, but I guess not. And then at the very end it turns Heartwarming when George actually kisses the thing, he's so happy that he has his life back.
The High School principal helplessly trying to regain some order after the floor cover of the gym pool was opened during the dance, until he just gives up with "Oh well" and dives in with the students. The look on his peers' faces at their colleague giving in to the chaos is priceless.