Personally I am not too impressed with CGN. It has an open air terrace with high often dirty windows.

Movements can be slow. It has Germanwings, Hapag Lloyd Express and not much else really. BA, AF, KLM, LH all have a handfull of flights. Freight wise this airport has loads of flights, in particular UPS and DHL. Problem is that most of those are during the evening/night when it is too dark.

If you got a full day I’d suggest taking the train to DUS and go spotting there.