The "Goodbye" challenge requires that you fall more than 30 ft to your death. The only place I've found where this is possible is the well on Seatown. I've fallen to my death plenty of times, but only got the challenge at the well. Are there other locations where it is possible?
You can also jump off the side of the building in the map Overwatch.
Or the bottom part of the map Piazza. Near Domination B Flag.
You can jump off the bridge or the path underneath the main monastery in the Sanctuary. New map that was part of the April Elite DLC on Xbox 360.
They are part of the DLC Collection 1 (only available on Xbox 360 right now) or with your Elite premium subscription (if you have one).