白根山 (Mount Shirane) and 草津温泉 (Kusatsu-onsen) are both in 群馬県 (province of Gunma). The most memorable place there may be the incredibly emerald-green crater lake 湯釜 (Yugama) at 白根山 (Mount Shirane). In the town center of the nearby 草津温泉 (Kusatsu-onsen), the same water can be seen in what is called a 湯畑 (Yu-batake).
The best way to go to 白根山 (Mount Shirane) and 草津温泉 (Kusatsu-onsen) is by car. However this can be a very long trip from e.g. 東京 (Tokyo).
Travel tips: Do not miss the crater lake 湯釜 (Yugama) of 白根山 (Mount Shirane). Enjoy the show featuring a traditional dance called 湯もみと踊り (Yu-momito-odori) of 草津温泉 (Kusatsu-onsen) then relax at a local hot spring, for example 草津大滝乃湯 (Otaki-no-yu).
Please check the photos of my visit in May 2007.