LH Business Lounge in DUS is nothing to write home about.
Why travel to Cologne by train and even to Duisburg(? sorry for the Duisburger! but a ghost town to me) when you can enjoy 5 hours in DUS with no sweat?
I would recommend that you go to the Altstadt(old town) with an S-bahn and a tram via HBF(Central Station), where you can enjoy local Altbier and stroll around near-by Koenigsallee where fancy shops are located.
In Altstadt, Brauerei Im Füchschen Peter König in Ratinger Straße 28 is a must for good beers and food.
Counting journey time with an S-Bahn from the DUS airport to the city and back, you can kill 5 hours quickly this way.
So, don't go to Köln, visit Düsseldorf.
I will have a 5 hr. layover in Dusseldorf (LH/LH connection).
Is it worth to travel by train to Cologne or Duisburg (?)walk around the town and have a dinner there or there are some attractions around the airport You can recommend (seating in LH Business lounge doesn't count as local attraction).
Many thanks,
Raf