My apologies for not having seen your posting until now, having just returned hours ago from a trip to Europe taking in the Paris Air Show plus a few airports.
There is a viewing deck on level 4 of Terminal One - this is ideally an afternoon photography spot with a view of aircraft taxying on the two parallel taxyways and also on runway 21R.
The only possibility of an outside location is on the northern approach where there are several pedestrian bridges spanning the Paholyothin Road. At these locations, you must contend with the following elements:
- fairly intense heat in August as there is no shelter
- encounters with police (on quite a few occasions, it is the locals who alerted the police to photographers' presence). There is no way to prevent this from happening, except to keep the camera discreetly in the bag until ready to shoot.