I did a day of photography at CGN
in August 2003 and I was quite happy with it! The observation terrace is open the whole day and gives good views of the cargo apron, the old and the new terminal and one of the 3 runways ( can't remember which one )
As Peter said before, the airport is dominated by Germanwings and HLX these days, but there is also lots of charter traffic like LTU, Condor, Hapag Lloyd and some international carriers, especially during the summer!
Other carriers like Dba, Lufthansa, British Airways, etc are there too!
Not to forget lots of UPS, DHL, LH
World Cargo and other cargo carriers - If you are lucky you can also get some IL
-76s or AN
You will arrive at the old terminal as you fly Germainwings, so it will take you only a few steps up to the viewing deck!