Addis Ababa is nice ... And you can see Lucy in the museum . its is country where human ancestors are begin. One of the ancient countries in the world.
If you have time , you better go outside addis a lot of historical things to see like :- ancient monuments ...Lalibela, Axum ...
Or wanna relax:- go to Awash river( Awasa langano rivers)
i think you need mini 10 days or more.
In addis there are tore guide office they will take you and show you where you wanna go. Make your you have enough money ....its nit cheap place ...
But defiantly amazing