When you go to Nikko or Kinugawa, why don’t you stop by Ryuokyo (龍王峡) for hiking? You can take the Tobu Line to Ryuokyo station. It’s about 50 minutes from Nikko and only 15 minutes from Kinugawa-onsen station.
NIKKO PASS world heritage area covers to Shinfujiwara station which is just one stop before Ryuokyo, so you could pay extra 190 JPY to go to Ryuokyo station. http://www.tobu.co.jp/foreign/en/pass/twoday.html
If you don’t have much time, you could just visit Nijimi-no-taki (虹見の滝). “Niji” means “rainbow” and “mi” means “see.” And “taki” means “waterfall.” If the light condition is good, you may be able to capture very beautiful rainbows (prisms) here.
Here is the official hiking map.
Currently, English map isn’t available, but hope you could get some idea.
Ryuokyo station (龍王峡駅) is at the bottom of the map (marked yellow). What I often do is to walk approx. 45 minutes to Musasabi-bashi bridge (むささび橋), go to the other side of the river, and come back to the station.
There are some shops and restaurants near Ryuokyo station.
It’s a good idea to come here before or after visiting Nikko and enjoy hot spring resort in Kinugawa.