Nikko (日光, Nikkō) is a small town in Tochigi Prefecture that is known for being a gateway to Nikko National Park (日光国立公園, Nikkō Kokuritsu Kōen). Because of this Nikko is usually used to refer to the region that includes Nikko, the town, and Okunikko, the area to the west, an expanse of forest, mountains, waterfalls, and hiking trails. Nikko town is also the home of one of Japan’s most famous shrines: Nikko Toshogu Shrine where Tokugawa Ieyasu rests.
The region is particularly popular in autumn when the leaves change color. This begins in late September around Ryuzu Waterfall and lasts until early November nearer Nikko town, but October is typically regarded as the best month to visit and so those planning on travelling to Nikko during this time are advised to book ahead.
Given that many of the attractions in the Nikko region are spread out over a wide area, car is by far the most convenient way to get around. However, public transport runs throughout the area to cater for the many tourists who visit by train, with buses departing at regular intervals. You can see the Tobu Bus time table here.
The Nikko All Area Pass offers unlimited travel on designated trains and buses within Nikko, as well as discounts at certain shops in the region. A two-day pass costs 4,520 yen for adults and 1,150 yen for children during summer (20 April to 30 November) and 4,150 yen for adults and 1,040 yen for children at other times of the year (see here for details). Tobu Railways also offers the Nikko World Heritage Area Pass for 2,000 yen for adults and 600 yen for children (see here for details). The key difference is that Nikko World Heritage Area Pass does not cover the Okunikko area—just Nikko town and the Tobu railway line which goes towards Kinugawa Onsen. Both of these passes can be purchased at Tobu Railways offices in Asakusa or Ikebukuro, as well as some travel agencies.