When trying to come up with synonyms for the phrase "doest off top of your head," it can be helpful to think about other ways to say "thinking quickly" or "coming up with something on the spot." Some possible synonyms might include "spontaneous," "improvised," "impromptu," "off the cuff," "unrehearsed," "unplanned," or "ad-libbed." Other phrases that could be used in place of "doest off top of your head" might include "thinking on your feet," "winging it," or "flying by the seat of your pants." Ultimately, the best synonym will depend on the context in which the phrase is being used, as well as the tone and style of the speaker or writer.