This game can be as simple or as complicated as your kids can handle. You will pick an item, or an attribute of an item, and ask your kids to find 100 of them (or however many you like depending on how long you'll be at the store). If your kids are young, then look for simple characteristics, like shapes, colors or patterns. For example, have them find 100 items with red letters on the packaging.
If your kids are a bit older, make it a little more complicated by throwing some math in there, too. Have them search for 100 items, with 25 percent of them containing all-natural ingredients, 25 percent containing a picture of the sun on the packaging and 50 percent containing chicken in the ingredients. Of course, what you have them look for is completely up to you.