The word "pre-arranging" implies planning and organizing ahead of time. Its antonyms would therefore be terms that suggest spontaneity or lack of preparation. Some opposite words are "improvising," "winging it," "ad-libbing," "spur of the moment," or "unplanned." While pre-arranging might be necessary for certain events and occasions, sometimes allowing things to unfold more naturally and without control can lead to more exciting and unpredictable outcomes. It is important to strike a balance between being prepared and being open to surprises, whether it be in everyday life or more significant milestones.