Answer: The old Granville house
Earlier in the film, both George and Mary had thrown rocks through the windows of the old Granville house; George had told Mary that if you broke a window, you got to make a wish. Mary made a wish, but wouldn't tell George what it was.
Later, after saving the Building and Loan, George receives a phone call telling him to come to 320 Sycamore, the address of the old Granville house, where he and Mary had thrown rocks through the windows on their way home from the high school dance. Bert and Ernie are putting up travel posters when George arrives. When he enters, Mary is waiting for him with a table set and chickens cooking in the fireplace. Bert and Ernie serenade the newlyweds by singing "I Love You, Truly." Mary tells George that when she broke the window years before, "This is what I wished for."