Blueberries. Don't put your freezer at -10 degrees F or something, just go with 0~-3 F. It helps things defrost slightly faster too. If the freezer is overly cold, the blueberry skins with strip off easily if eaten and will remain hard and icy for a considerable amount of time.
If you like having sweets in your cereal such as chocolate chips or cookie shreds (like me :3), try putting in a goodly amount of frozen blueberries instead. It'll freeze the milk around it, making for a nice and tasty treat. They're a bit icy in my experience, but still quite good.
This sounds awesome! I'll try it out when I have a chance. Do you have a recommendation for the amount/ratio of Cocoa to Sugar to butter?