1Terminal area
The civil terminal offers good views of the airport, both from outside and inside the terminal. Aircraft parked on the civil apron can be read off easily. Military choppers and transports on the opposite side of the runway require a good set of binoculars. Photography is not an option here because of backlight.
2Approach 29 and east side military apron
When leaving the civil terminal, continue towards the city centre. Where the railroad bends right beside the main road, an exit to the left can be found. Turn immediately right again, this road leads a bit uphill. Continue till there's a dirt parking lot on the left side of the road. From here you can have a look at the transport aircraft parked east of the military apron. In theory, you will be able to see aircraft on approach for runway 29 but you will find yourself right under the approach path, so again, photography is not an option here.
3Approach 11
If you really want to make pictures at Niš and have some time, spot 3 might be worth trying, but is terribly hard to find without proper navigation equipment. Probably the best thing to do is try to round the airfield to the south side, as depicted in the map, more or less following the railroad tracks to the west. Cross the railroad south of the huge railway emplacement. Turn left at the first intersection, then right again at the main intersection in the village. Head north and just before you reach the railroadtrack again, turn right. This spot provides great views of aircraft on approach for runway 11, although the airfield cannot be seen from here.