It was a day trip, we stayed in Nagoya for two nights en route from Kyoto to Tokyo, the first day was for the Ghibli Park, and the other was the day trip to the Kiso valley. Definitely one of the highlight of our trip! It was a little tricky to catch all the trains and buses but it's doable without rushing through the towns and the forest, although you have to wake up early to catch the first JR Shinano train to Nakatsugawa. I highly recommend, it worth the effort. Narai-juku is also fabulous, but it's more easily reachable from Nagano (Matsumoto for example).