Situated 30 minutes by car from Shibukawa, Mount Haruna is one of the famous mountains in the Gunma prefecture.
Mount Haruna is a place I like to go often. As I live not far away from there, I can definitely say that the best season to go is during the fall. During this time of the year, the colors on the mountain are amazing, going from light yellow to orange to dark red. During the Christmas season, there is a lighting festival which I recommend as well. In Winter, the lake freezes and in spring, there is some cherry blossom to see as well. There are activities all year long for children and for adults as each season has its perks at Mount Haruna.
It is quiet, not busy (if you avoid Sundays) and it is a perfect place for a short hike, a bicycle ride, or even fishing on the lake. Often, there are bikers or car lovers during weekends, as well as cyclists going up the mountain. Renting small boats or pedal boats to go out on the lake is possible too.
If you have time (and money as the cable car ride is pretty expensive - it is about 1600 yen per person), there is a small shrine at the top, Haruna-fujisan Shrine, and you can admire a great view over the lake and the surroundings. If the sky is clear, you can see as far as Maebashi.
If you are travelling by train and have a bike, you can stop at Gobara station and go up to Mount Haruna by bicycle. Otherwise, from Agatsuma station, it is about 30 minutes ride by car. On the way, there are a few farms where you can buy local fruits such as apples, blueberries or strawberries.
On your way to Kusatsu, Mount Haruna is a must to see at any time of the year.