Extemporarily is a word that is often used to describe something done without prior preparation. If you are looking for synonyms for this word, you might consider any of the following: spontaneously, impromptu, off-the-cuff, without planning, ad hoc, improv, spur of the moment, on the spot, unprepared, unscripted, unrehearsed, or unplanned. All of these words capture the essence of extemporarily, and can be used in a variety of contexts. Whether you are describing a speech given without notes, a party planned at the last minute, or a decision made in the heat of the moment, these synonyms are sure to come in handy.