I live not that far away from cologne. I know some courses here in the area (but not all of them). I can recommend to play in Dortmund and Lünen which is a little drive of an 1 and 1/2 hour from cologne. I don't know of any course in cologne. The nearest course should be in Neuss (maybe about 50km from cologne). But the last time I was there it was missing proper tee signs and a course plan.
In Dortmund you got a nice diversified course with all sort of holes (open shots, double-mando through trees, over water, along water etc.) Here is a plan for that course:
http://gw-kley.de/discgolf/discgolf-wischlingen/
In Lünen you got 2 seperate courses ("Seepark" and "Alte Ziegelei") with 12 holes each. "Seepark" has shorter but more technical holes. They are all very nearby and if you play Solo a round will be around 30 minutes.
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It has some nice "over-creek-shots" and some death putt-holes.
"Ziegelei" has more open holes and they are all longer.
There are also two courses east of cologne (dont know how long you drive there...but must be around 1hour also) where I have not been yet. "Bad Fredeburg" and "Meinerzhagen".
Here you got a map of all courses in Germany:
http://www.discgolf.de/parcours/#map_top
I think you can find every of these courses here at DGCR (database).
If you have more questions, you can sure ask me
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