Rund J
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You can take the Bus to a station called " Rheinturm " and from there walk down to Gehry Builduings "Gehry Bauten" . Interesting, funny architecture and worth a visit.
Continue the walk down to Old city, it takes you around 30 min walking.
Best visited on a Weekend. I recommend buying a 24 hours public transport ticket in Dusseldorf ( 7.60 Euro ) to enjoy the whole city for one day and plan in advance the places to visit.