Mainly off the top of my head. I rely on a map for general inspiration, but do not usually have any real notes at all, only pretend ones to make the players think the adventure has been planned. Of course my old-time regulars knew that wasn't the case, especially when they were returning to Greyhawk from the Land of Chin. as well as when dungeoneering, as all of them had seen the encounter notes page for one or more levels, single lines for each of 20 or so places on each level.
Dice assisted in that by indicating random encounters, what was encountered...if it fitted. Otherwise I would select the one that seemed right or make up something.