At ADD, stay airside—Bole is a major hub with long queues, making lounges and cafés the best use of time.
A 3-6 hour layover in Addis Ababa is very practical - Bole Airport is only 5km from the city center. A taxi costs $3-5 and takes 10-15 minutes. Head to the Tomoca coffee shop (established 1953, the city's most famous roaster) on Wavel Street for a traditional Ethiopian coffee ceremony experience. The surrounding Piazza neighborhood has colonial-era Italian architecture worth walking. Allow 60 minutes to return to the airport.
Six to ten hours gives you time for the National Museum and the coffee circuit. Start at the National Museum of Ethiopia (10 min from airport): see Lucy (the 3.2-million-year-old Australopithecus skeleton) and the outstanding ethnographic collection. Walk to Holy Trinity Cathedral (Selassie Bet) nearby - Haile Selassie's burial site, with extraordinary stained glass and Ethiopian Orthodox atmosphere. For lunch: try kitfo (Ethiopian beef tartare with spiced butter and cheese) or a full injera spread with tibs (sauteed meat) and various lentil wats. Return via taxi - allow 45 minutes for the airport.
With 10+ hours, Addis opens up considerably. Mercato, Africa's largest open-air market, covers several square kilometers of stalls selling everything from spices to electronics - chaotic and fascinating, best visited with a local guide or hotel-arranged trip for orientation. The Ethnological Museum inside Addis Ababa University (formerly Haile Selassie's palace) has excellent collections and sits in beautiful grounds. The Entoto Hills on the edge of the city offer views over the entire Addis basin from 3,200m altitude. Ethiopian Airlines passengers can book free city tours via the airline's transit tourism program - check at the transit desk on arrival.