See Lake Onuma-Ike
Lake Onuma (大沼池） in the Shiga-kogen Highlands is the largest lake in the area with a circumference of 5.5 kilometers. At an altitude of 1730 meter, the water of Onuma is incredibly blue. You won’t find any fish in the water due to the high acidity of the lake (pH4) as it is a volcanic lake. Due to this it is also not advised to swim in it as contact with the water might irritate your skin. Another beautiful feature of the lake is the torii gate standing in the water. This one may not be as big as the one in Miyajima, Utsukushima Shrine, but it still gives one quite an impression.
If you feel like having a relaxing day off bring a picnic blanket with you and enjoy a nice lunch while enjoying the blueness of the lake.