Lalibela
Your time is a little to short for traveling to the historical sites outside Addis. So I recomand you to visit the rock carved church Washa Mikael in Addis, the athletes running sites Sululta or the forest behind Ararat hotel (amazing trail course where I met Genzebe Dibaba and her younger sister last Sunday) and go to public places like Piassa, small coffee shops, Cultural restaurant Yod Abessinia, to meet the charming people of Ethiopia. Maybe we meet at the GER or the pasta party the night before. Have fun anyway and come back!