I know the какой-нибудь is the point of the exercise, but doesn't the fact that the food is in the genitive case already indicate 'some' (an unspecified quantity - partitive genitive)?
купи мне сахара - buy me some sugar Хочешь воды? - do you want some water?
I understand the partitive isn't as popular as it was, but it's still used in everyday Russian. Worth a mention in the notes somewhere?
Partitive means "part of a greater whole" - it's indeterminate, but it means an amount with limits, usually set by custom and usage between the speaker and the person being spoken to - missing context here. You ask your spouse or roommate, "Get some bread on your way home", the person will understand that that probably means "get the usual amount of bread", such as a loaf. The kind of bread might also be understood. If my wife asked me to bring some bread home, I definitely would not buy pumpernickel, or rye, or even whole wheat. I'd get some sort of white bread. Or I'd ask, "What kind of bread do you want?"
In my view the use of the partative genitive already means 'some'. One other way of saying that without the genitive would be 'какой-нибудь хлеб', however as has already been shown, 'какого-нибудь хлеба' could also be considered correct, particularly for emphasis, like 'give me some bread, any bread!' The bottom line is that both accusative and genitive versions are both fine and would be understood. That's the important thing. :)