Answer:
"In Köln war ich am Flugplatz" or "Ich war in Köln am Flugplatz"
Step-by-step explanation:
You could actually also use "Strand", or "Meer", just there is no beach or sea in Cologne.
Or you could also say:
"Ich war in Köln am Flugplatz."
It basically means the same, just formulated differently.
Hope that helped