The word "preplanned" implies that something has been planned in advance, often meticulously and with great attention to detail. To find the antonyms of "preplanned," we can look for words that convey the opposite meaning: spontaneous, unplanned, impromptu, unpremeditated, ad hoc, extemporaneous. These words suggest that something is done on the spur of the moment, without prior thought or preparation. They may connote a sense of unpredictability, chaos, and lack of control. While preplanning can be beneficial in certain situations, being able to adapt to unexpected changes can also be an important trait in life.