Hi. Not sure how much you want to do really as this one is a bit of a trek East but if you can manage it Harar is worth a visit. I did it last year, and I recall it being about 240 miles or so, a real step back in time and some great hospitality. I think there is a train but it doesn't go to Harar but somewhere nearby.
I had a guide and we drove, as I had some business there, but managed to get in the hyena feeding ritual and a bit of a poke around town. Old walled city but it's on the way to Somalia, for which I think you will need a bit of a sense of adventure.
In Addis I use the Sheraton or Raddison blu (for points). I'm there around November time too and want to visit the museums this time round in my free time.
Ethiopia has really come on in the last 19 years since the Derg, and is really becoming an economic powerhouse with the ruling socialist party's policy of reinvesting 70% of Government revenues into the country. If using Internet / sms, just be a little wary if sending sensitive information as communications (Goverment controlled / owned) are monitored...
Tipping is expected, not US crazy amounts but I do little and often, get small 1 and 5 Birr notes. Beware some of the money changers outside Addis. There are some rules about currency changing / exchange rates but I can't exactly recall what they were now. You can get a tourist VOA for about $50 which seems to be the going rate in parts of Africa these days, I get mine in advance as is for business.
Maybe buy Lonely Planet or something like that which would be a sound investment. Good luck and enjoy it.