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Don't know any good places at AZO or FNT, but at DTW i can give you a few spots. If your willing to spend the buck per hour hit the decks. The parking deck is my favorite place. Take it up to the 6th floor (the top) and drive to the far noth part. You have a great view of the string of pearls when the planes are landing on 21R, 21C, 21L . go the south if they are or 3R 3C 3L. It is great viewing from up there. Also if you go back by the car rental lots follow the signs saying car rental return. It puts you 500ft in front of 21C. I think there is a park to the south of the airport that may be good for viewing never been there. Also try by the fed ex building i was there once and your close to the action.
Best of luck and enjoy DTW.