Well Mrs AITF and I have recently returned from our splendid two-week holiday in Ethiopia, so I hope I can help!
If you have five days then I would suggest you either do a North tour or a South tour (we did both since we had bit longer).
If you opt for North then consider Gonder, Simien Mountains, Axum and Lalibela (the latter is a must in order to see the rock-hewn churches and the fantsatic scenery and countryside).
If you go South then think about Arba Minch, Awassa and the region around Lake Langano.
Distances are massive and road quality and traffic varies a lot. We decided to use internal flights with ET to avoid long uncomfortable drives between places and this worked out to be very comfortable.
We were very lucky with guides we had and I can highly recommend both of them- I will send you a PM with their contact details.