At night there's a clock at the side of the Rhine tower. Have a look in the evening and try to figure out how it works. I understood it but I'm not telling you - it's more fun to find out yourself! Little hint: This picture was taken at 6.49 p.m., 14 seconds before 6:50 p.m.!
A area which not many tourists find is the habour quarter. Close to it you find the Gehry buildings, WDR broadcast, the Norhtrhine-Westfalia Parliament, the "City Gate" in which the city hall has it's place and of course many other architecual interesting buildings. If the weather is fine it's a really nice route for a walk around.
You can reach the quarter with walking distance from the Oldtown, with the tram 708 or bus 725 from the main station to Franziusstraße.