5 days... you are massively underestimating the time - you will be spending most of your time in the car particularly if you want to see scenic routes. Road are often quite narrow with little scope for overtaking. On mountain roads - in many places you will struggle to go more than 50km / hour in a car. We took a motor biking holiday up the Sanriku Coast several years ago and two days to go from Matsushima to Kuji was seriously rushed.
I would rethink your time frame, rethink your area or be content to spend a lot of time of expressways. (The expressway on the Japan Sea side is only one car in each direction and can be painfully slow.)
Is there are reason for Koriyama being your starting point? Sendai would make more sense. Yamadera is probably more easily done by train - if you like walking / nature get off at the station before and walk.
The Japan Sea Coast is great driving. The coast on the Pacific side in someways is more beautiful - despite the tsunami damage - but the road doesn't run directly along the coast and you need to stop and get out of the car to look at places. Kita yamazaki cliff? is impresive (between Miyako and Kuji) but the coastline is dotted with natural beauty that the tsunami hasn't taken away.
I haven't been to Osorezan but I've only heard good things about it.
Highlights of Tohoku for me that I have been to include:
- Towada ko,
- Japan Sea Coast - Fukaura is not a bad place to stop & has an interesting temple.
- Shirakami sanchi - the road between Hirosaki and the coast that goes through Shirakami would definitely be on my list (I've never been there at the right time of the year).
- Sakata on the Japan Sea Coast (I haven't been to Tobishima near there but would like to next time I'm in the area)
- Chokkai san, Gassan. (I haven't been to Haguro san)
- Jodogahama, Kitayamazaki falls and lots of places on the Pacific side I don't remember the names of.
- Goshogake / Tamagawa / Hachimantai - beautiful nature, volcanic activity.
- Zao san, for a caldera mountain.
- Hachirogata - not because it's beautiful, but because it seems like such a terrible waste to have filled in a lake to make land for rice that isn't being grown much anymore.
If you are on country roads, and want to go to quite off the beaten places like Tehaizaka or Tahoin a GPS will be a big help
http://ponkanchan.blogspot.jp/2011/05/road-much-less-travelled-tehaiza...