When it comes to the phrase "making up off the top of one's head," the antonyms are words and phrases that are used to describe the opposite of creativity, spontaneity, and improvisation. To begin with, the words "premeditated," "rehearsed," and "planned" can all be used to describe an approach that is the exact opposite of coming up with ideas on the spot. Other antonyms include descriptors such as "methodical," "calculating," and "formulaic," all of which refer to an approach that is much more planned and structured. In short, the antonyms of "making up off the top of one's head" all point towards an approach that is intentional and carefully thought out rather than spontaneous and creative.