When trying to recall information or coming up with something quickly, people may use the phrase "made up off the top of my head." However, there are a variety of synonyms that can be used in its place. Some interchangeable phrases could be "off the cuff," "spur of the moment," "impromptu," or "without prior planning." Other synonyms could include "ad hoc," "unrehearsed," "unplanned," or simply "on the fly." These expressions are often used to describe something that is done quickly or without much preparation, but can also imply a certain level of creativity or spontaneity. Whatever term is chosen, it can convey the idea of spur-of-the-moment thinking or improvisation.