I remember the baking-powder submarines; one of the few cereal-box enclosures that ever engaged my attention.

"When was the last year that a cardboard cutout toy was printed on a cereal box?"

I remember masks being printed on the back of certain cereal boxes; can't remember the brand(s) of cereal or the characters represented by the masks, but I do remember that the face occupied the entire back of the box plus a small portion of each side panel. The result was a mask just large enough to completely cover a child's face. Cutting out the eyeholes was the trickiest part of putting them to use.