A brief stop at Kegon Waterfall in Nikko
Up a windy mountain road, you'll come to Kegon Waterfall.
Kegon Waterfall is 100 meters high, and is considered one of Japan's three most beautiful waterfalls.
The elevators to the lower observation deck are open from 8 AM to 5 PM, with a 530-yen admission fee. Access is best by bus from the Nikko train stations (JR or Tobu), and takes about 50 minutes - get off at the Chuzenjiko Onsen stop.
The one-way ride is a steep 1100 yen, though a 2-day bus pass is 2000 yen. A great half-day-or-longer trip would add the hike through Senjogohara Marshland, as the bus that stops at the waterfall continues on to Yumoto Onsen (a good starting point for the long hike).
Note that a pass that goes all the way to Yumoto Onsen will probably be more expensive, though my 4400-yen pass included round trip transportation from Asakusa and four days of bus and train rides around Nikko. Details about different pass options can be found at Japan-Guide's website.
And speaking of multi-day passes at Nikko, I think it's easy to spend at least three days there visiting the shrines and temples, natural sights like Senjogohara and Kegon, and the two "theme parks" in Kinugawa. Those looking to relax and take things slower or enjoy the hot springs will easily spend 4-5 days in the area!