Yes, we buy cold cereal. It may not be frugal but it's part of our life. We do try to stick to the healthier/cheaper varieties, and there are certain sugar limits beyond which Mama Squirrel will not budge.
What do you do with the end-of-the-box crumbles and bits? I save them in a plastic container and use them for baking...usually pulverized in the food processor. Works very well in dried-fruit balls and chocolate-raisin balls, but you can also use them in muffins or oatmeal cookies. I figure we paid for that shredded and puffed stuff, so we may as well use every bit of it up.
What do you do with the boxes? You can use the cardboard for gluing patterns to. You can cut them down a bit to make magazine holders. But if you really want to see how good cereal boxes can look, check out this link at Frugal Family Fun. Wow!