The cereal left in the bottom of the box is usually not enough for a serving and often consists of crumbs and broken pieces. To avoid waste I have been adding the unsweetened cereal bits to meat loaf/meat balls and using the sweeter bits n pieces in muffin mix and cookie batter.
Rather than pull out the food processor or blender you could process these small amounts of cereal in your coffee grinder if you have one. Or what I usually do - put them in a plastic bag, seal and then use a rolling pin to crush them up.
Easy peasy lemon squeezy.