Situated in the southwestern part of Nagano Prefecture, this area was once a section of the old Nakasendo Highway, one of Japan's historic transport arteries joining Kyoto with Edo (present-day Tokyo). Roughly following the Kiso River, the once-important post towns now form a well-preserved living museum of the Edo Period, with most modern facilities being hidden from sight.
Check out the JNTO Kiso Valley Guide
The centrally-located train station for the Kiso Valley is Kiso Fukushima (木曽福島), located on the JR Chuo Main Line.
There are several different approaches to Kiso-Fukushima from Tokyo. One route is to take the Tokaido Shinkansen Nozomi to Nagoya, then transfer to the Wide View Shinano for the run to Kiso Fukushima (3 hours 20 minutes, ¥13800).
Another route is to take the Nagano Shinkansen Asama to Nagano and transfer there to the Wide View Shinano. This takes about 3 hours 40 minutes and costs ¥11300.
If you use the Japan Rail Pass, you should go via Nagano or Shiojiri.
From western Japan, including Kyoto and Osaka, take the Shinkansen to Nagoya and change to the Wide View Shinano or to local service.
Note that some other parts of the valley are located at other stations near Kiso Fukushima on the JR Chuo Main Line (see below).
From Tokyo you can also sign up with local motorcoach tours for a one or two-day excursion to the Kiso Valley region.
There is a historical section of the Nakasendo Highway through the Kiso Valley.
The Kiso valley is known for the "Hinoki" fine-grained cedar pine trees (Chamaecyparis Obtusa). This scented wood is known for its durability. Hinoki is used for example for bath tubs and accessories. Japan's most important shrine, Ise Jingu is rebuilt each 20 years using Kiso Hinoki. You can buy Hinoki goods, especially around Narai. North of Narai, visit Kiso-Hirasawa (木曽平沢) which is famous for its lacquerware.
Take your time to explore the valley and overnight in an historical building at one of the minshukus.
Kiso Valley has also several (small) ski resorts. WikiPedia:Kiso District, Nagano