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RDU (between Raleigh and Durham, NC) has one: https://www.rdu.com/observation-park/

also conveniently the best spot to wait to pick up an arrival (as they like to keep traffic flowing outside the terminal)

Manchester airport has a viewing park along with a Concorde in a hangar you can walk around. (I’ll take any opportunity to plug a visit to Manchester)

Yeah, I don't know. Krakow had one huge observation deck, now it's all gone and there isn't a single area of the airport which would have the view of the runway(not without going through security first).

I think BWI has one if you're in the right spot. It was closed the last time I was there, however.

Yeah, there's one at DUS, with security control and indeed a fee.