For a day by the water with a difference, check out the bubbling pools of Otonashinoyu. This place is known for its mineral-rich thermal waters.
Situated in Chino, this naturally heated water source is well worth a look. But you'll find lots more to explore in this area. Lake Shirakaba, Tateshina Lake, and Kirigamine Highland aren't too far from Otonashinoyu.