there´s an observation deck in terminal one as daniel pointed out. actually there are two, on each side of the terminal. however, if you want to take photos you either have to rearrange the benches since only standing on these you can overlook the glass panes.
there are two other nice places but you must be willing to go by foot for quite a while (around most of the airport), i assume you arrive at CGN by plane so you won´t have a car.
when you´re on approach to runway 14L there is a horse racing track to your right, and that´s a very beautiful spot for watching the aircraft passing by very low. i´ve sat there often this summer and it´s a treat, especially at night.
but it might not be fun in winter, of course.
the 2nd one is on the eastern end of the airport, close to the landing threshold of the east/west runway (don´t know the designation). but there isn´t that much traffic.
at night, from there you´ve got a wonderful sight on to the entire airport and its nicely illuminated cargo facilities.
If only pure sweetness was offered, why's this bitter taste left in my mouth.