In the center of Cologne's busiest shopping street is Galeria Kaufhof which is good for shopping but better for seeing the city. On the top of the buildings parking lot (accessible by elevator) is an incredible view of Cologne from the city center.
Views are scarce from the west side of the Rhein making this an appealing place to visit. Not only that, but it's free. Head up there for a beautiful sunset over the city and it's surroundings. You can even shop on the way up and down.
Occasionally, there will be summer parties up there. Check one out!