A good spot within walking distance would be the bridge over the A-4 (and airport roads).
Walk towards and past the parking garage P2
from the terminal (left when you exit the terminal through the arrivals hall) towards the "cargo route".
Past the gas station and continue on until you see the bridge.
If the 36C is in use for departures (or 18C arrivals, but then not everyone will take it) aircraft will taxi past there. Never shot from there (usually drive to the 06-24 or 36R, but that might be too far for you to walk and there are no busses), but it looks like a good spot.
If the 09-27 is in use during the morning or early afternoon, the McD is also a good spot. Take the shuttlebus (free) to P3 (longterm parking) and walk a short distance towards the golden arches