The other day I went to Shiga Kogen Heights and walked on the top of not active volcano - Mt. Shirane.
It takes around 20min to climb the mount from Parking place. Entrance fee is included in the Parking fee 420yen/car.
It’s good exercise and after you get sweat a bit, the beauty is just right infront of you.
Volcano is full of beautiful light blue/white onsen. You can take incredible pictures of the volcano and view of the Shiga Kogen mountains around.
 

How to get there?
There is also local bus runing there from Yudanaka-Shibu Onsen and it takes  1h 10min.

One way Bus ticket is 1500yen.

Yudanaka / Shibu Onsen / Mt. Shirane   / Back
  09:10             09:14             10:20           11:30
  10:00             10:04             11:10           13:10
  11:45             11:49             12:55           14:35
  13:10             13:14             14:20           15:55
 

I was told the water temperature is aprox. 20-25 *C and I can not get very close to the water, bcs there is lot of gas coming from the lake, which is not good for the health and body. You can not smell it, but just suddenly collapse. So Mt. Shirane lake is beautiful, but dangerouse from the other site. I definitely do not recommed to anybody to bath there, or get closer than its permited.
Anyway this place is very famous and every year there are tousents of visitors.