"I do not mix fruit and vegetables."
Translation:Nie mieszam owoców i warzyw.
If you simply "mix something" (only one object, like soup), then it's Accusative: Mieszam zupę.
If you mix "X and Y"... it's also Accusative. Mieszam owoce i warzywa. So Genitive here is only because of the negation.
You can also mix "X with Y". That's Accusative for X and Instrumental for Y: Mieszam owoce z warzywami (and Genitive for X, Instrumental for Y in negation)