The phrase "Willy nilly" is used to describe a situation or action that is done without any particular plan or order. The antonyms for this phrase would be "methodical," "planned," "organized," and "systematic." These words all suggest a deliberate and intentional approach to something, as opposed to the haphazard and random quality of "Willy nilly." Other antonyms might include "purposeful," "prepared," "calculated," and "structured." All of these words imply that there is a clear goal or objective in mind, and that the actions being taken are designed to achieve that goal in a thoughtful and intentional way.