I'm planning a trip to:
Seoul (Sept 20 to Sept 27),
Kyoto (Sept 27 to Oct 3),
Tokyo (Oct 3 to Oct 9).
I know it's not the best season to visit, but is it ok? I would do a lot of day-trips around those places.
That's a very good season for weather. Will be warm during the day, but should cool off nicely at night. Probably many clear days.