This was a day we'll never forget. We took a little train up from Nagano for the day to see the famous monkey park in the Nagano mountains. After the train ride you have to catch a ride to a bus-stop near the trailhead, then hike up for about 40 minutes to get to the park. The remoteness of the site is a good thing though, because once there you may walk freely among the little macaques. At times they surround you.
Along a river there was an onsen, or naturally-heated bath, constructed specially for the little monkeys. There's a live monkey-cam if you want to see what the bath looks like right now.