We have just returned from a weekend at Lake Langano, about 200km south of Addis Ababa. The closest thing you can get to a beach resort in a landlocked country and a popular retreat from the chaos of Addis Ababa.
It is the only lake in Ethiopia that is free from bilharzia, so safe to swim in. Even though the water looks like tea. (Apparently there are hippos and crocodiles around but we saw no sign of them).
These cottages have been rented out by the British Embassy since the 70s. They are pretty basic but a perfect spot for a quiet and relaxing break.