The front of the box announces 1 in 10 specially marked boxes has a gift card inside! You could win a $5, $10, or $25 cash card from Discover.
I never noticed the box cover when I purchased my Cheerio's. I don't even remember which store I bought this box from. Sure enough, while pouring cereal we won this $5 Discover card!
Update: We went into the basement and pulled 2 more specially marked boxes and won 1 more $5 Discover card.