Dusseldorf is a three hour train drive from Frankfurt. In my opinion it is one of those small, big cities. Meaning it has a lot of things to offer but you don’t feel overwhelmed.
Downtown has a quaintness that is truly relaxing. You can stroll down the city streets and don’t feel like you are being moved through an assault course. Check out the city views from the Rheintum and the Schloss Benrath eightieth century palace is worth a tour. Also check out the Neandertal Museum and Kunst I’m Tunnel underground art museum. If you have the time try and squeeze in a boat cruise down the Rhine. I found it to be very enjoyable.
I stayed at Steigenberger Parkhotel on Koenigsallee. The suites are spacious and service throughout was German efficient. The balcony bar is awesome.
Hope you get to visit Dusseldorf.