Try the following tour, recommended by residents or metro enthusiasts:
There's a modern suspended monorail at the airport. Lines S1 or S7 take you to the city. You can see the older standard Düsseldorf Stadtbahn station type on lines U74-U79 between Nordstraße and Hauptbahnhof, the newer type between Hauptbahnhof and Oberbilk. A must-see is the century-old suspended monorail in the neighbouring city of Wuppertal. To catch it, take line S8 to Wuppertal-Hauptbahnhof or Wuppertal-Oberbarmen.