The Lonely Planet guidebook recommends an itinerary called "Ethiopia's Historical Circuit:" ...

 “The historical sights along this loop north of Addis Ababa are monumental in both scale and detail.  The journey, although spectacularly scenic, is monumental in itself, covering at least 2500km (1500 miles).” *

And so I followed the historical circuit, (not a loop, but more of a crisscross of my own choosing) from Addis to Axum, Lalibela, Gonder, and Bahir Dar.