First, sign into ornitho.de to make the most of local observations! You can search the database better after you've made a certain number of entries, so start reporting those pigeons and parakeets...
If you're not a member, you cannot see exact locations. Still, if you set it to English and go to "Summary of today" and then click NW (in red, for the state) followed by BM or K (in grey, for the district), you can check sightings per day. It will soon become clear what the better areas are.
I do not live near Cologne, but I've looked into some sites around the city to help out visitors before.
A good area near Cologne is the Wahner Heide (surrounding the airport) with e.g. Woodlark, Middle Spotted Woodpecker, later in the year Red-backed Shrike and Melodious Warbler.
The vineyards of Bad Honnef just south of Bonn are a good spot to search for Rock Bunting (northernmost spot in Europe). Some years ago, Cirl Bunting also bred there.
Southwest of Immendorf there are some gravel pits which hold a few birds, but I don't know how easily they can be viewed.
Probably the best spot in the area are the former sewage ponds just southeast of Bedburg in Kreis Bergheim. Should be easy to reach by train. They are visited by quite a few birders (nothing compared to the UK or the Netherlands) and turn up rarities with some regularity.
Hope this helps!
I will be sure to take a look at the sewage ponds they sounds like a decent gamble. 
