Well, I wouldn't want to spend 5 hrs in a DUS business lounge, and spending 5 hrs at DUS isn't much of a delight either (especially when you're travelling in the evening, the terminals will be quite empty). There's no reasonable restaurant at the airport (and I don't call the "Leonardo's" self-service, the Nordsee self-service and the McDonalds (guess what, self-service) eateries a proper restaurant), so I'd suggest you visit the city of Düsseldorf (a trip to cologne in 5 hours may be a bit too much, even if the ICE takes only about half an hour to get there from the airport, the trains don't run very regularly).
After you have picked up your luggage, proceed to the SkyTrain station above the departure floor (don't take the S-Bahn train leaving below concourse C as those local trains only depart twice an hour and take much longer to the central station) and buy a ticket for "A" fare (costs about 2 EUR, if you're accompanied by someone else, you should buy a 4-trip-ticket). When you arrive at the airport station, take any local train (red two-deck trains) leaving from platform 5 towards Düsseldorf, Koblenz or Aachen and get off about 5 minutes later at Düsseldorf Central Station. From there, proceed to the underground station and take any of the trams towards "Duisburg Meiderich" (U79), "Messe Nord" (U78), "Düsseldorf-Lörick" (U74) or "Am Seestern" (U77). Get off at "Heinrich-Heine-Allee" (I think it's the 4th station after Central) and you'll be right in the city center of Düsseldorf.
There, you'll find an aggregations of restaurants and bars that is considered to be the densest in the world, so you should not have any problems to spend quite some time (unless you are easily bothered by bachelor parties and huge crowds of people, especially on weekends). To get back to the airport, just take the trains the other way round. You should plan about 30 to 40 minutes to get back to the airport.