mwahahaha...Appliance boxes were definitely the best things in life when I was a kid. And, since I didn't have to wait for the folks to get a new refrigerator or range....'cause they had a small Eaton's store (similar to Sears...actually in later years when Eaton's closed their small shops, my parents had Sears, too)....anyway...the appliances for floor display would all come in huge heavy-duty cardboard crates...and after the crates were removed my younger brother and I had full-reign over what those crates became....houses, spaces ships, cars, trains, you name it! There is nothing quite like the imagination of a child.
Oh 'n to the person who mentioned cereal boxes...those aren't quite big enough to have fun with....although if you ever watch an infant/toddler at Xmas, you'll notice they tend to be more interested in the boxes than in the toys which came inside!