Okukinu is a fantastic area in Tochigi in between Nikko and Oze. This walk is a very good overnight trip. Though if are very fit you could do it in one long, long day.
Its a very nice drive to get to the area, though the road is a bit narrow in parts. There are lots of ryokan on the way in too so it could easily be made into a 3 day trip. You can't miss the starting area. There is a big car park and the road closes to regular traffic.
From the parking area its about one hour walk to the first of a small group of in park ryokan. You can take a bus if you prefer. The walk is nice, there is a bit of construction at the start but you will soon be past that.
We stayed at Hatsuchonoya Onsen. Its very nice and covered in a separate post.
Its a 2 1/2 hour walk to the top. There are some quite long steep sections but its beautiful along the way. At the top there is a great alpine meadow. Its a good place for lunch. From there you can walk to the top of a small mountain with nice views, but its not a must see. You can also walk to Oze, but that must be a very long walk.
Its a 3 1/2 hour walk back to the car park. You might consider taking the bus back from the onsen area. The whole walk starting from the onsen, finishing at the carpark and including the small mountain must be about 20 km. If you skip the mountain and take the bus its probably 12 or 13 km.