however, i must admit that if i have time, i will either make a map and note locations with significant sites and a timeline of significant events for reference purposes as the pc's wander around causing havoc and mayhem. it doesn't feel so much like chaos if i don't have as much planned. ^^
at the very least, i will review the material to see what's likely to be covered. and it's not unknown for me to take a quick 15 or 20 to go off and make up some stuff for when they really manage to get off the map.