duessoldorf 周邊既城市

day tour ins
www.phantasialand.de
or a day out(sunday)to the famous F1 race track Nurburgring.
Boat tours;
Duesseldorf-Cologne.Duesseldorf-Koenigswinter(beethoven,s Birthplace).Koblenz-Boppard or Ruedesheim-Mainz
www.k-d.com

Well, so many places to see in Germany(for me). Really depends how you know about Germany. For me, I like South Germany
If you do not speak German, tough to travel into small cities...Also, do not expect you can see many things than in a city that you mentioned above
How many days that you can spend? Do you want to travel by long distance train...?
Near Duesseldorf: Leverkusen - football team; Kerpen - Michael Schumacher. If you are not fans, no need to go...
Hameln: famous for the folk tale of The Pied Piper of Hameln