as i live in the general area (about 60 km north of cologne)
my suggestions for go to places:
"Bislicher insel"- a sidearm of the lower rhine.
Dingdener Heide"- meadowlands with shallow ponds
and some of the many gravelpits/quarry ponds in the general area of the lower-rhine.
in october the main attraction is the arrival of the numerous arctic geese.
hope that helps a bit for orientation ...