Hi Eva & Thomas,
Sounds like a great ride. If you don't mind me asking, are you starting in Addis to avoid Egypt and Sudan and their associated hassles? There typically aren't many people flying into Addis, but some certainly have done it. We met a couple of Austrian guys that shipped in and out, not much hassle but a bit expensive for shipping.
As for places, there are a great many, and every country is different. In general though, a fairly standard overland route exists but it gets busier and more well travelled the more south you go. Make sure to explore as much of Ethiopia as you can. The Omo valley is a great destination, and f you are up for some additoinal adventure, the Lake Turkhana route into Kenya is a classic, but can be tough going. Have a great trip.