The closest one is Jigokudani Monkey Park, features about 200 snow monkeys who play and keep warm in the onsens.
Park is really in the forest where are a lot of monkeys. You have to walk to get there 15 to 25min.
The walk tracks are trough the forest and are very enjoyable in each years period. You can meet some monkeys even before the entrance. They are just walking around or sitting on the trees.
Behind the entrace there are monkeys everywhere. You can get very close to them, take detail pictures or movie and observe the real monkey life.
It was very nice trip during my visit at Hakuba.