Lake Chuzenji is beautiful!
I highly recommend taking the ferry from one side of the lake to the other. The ride is very serene and having the wind in your hair as you sail past gorgeous mountain scenery is something that simply just needs to be experienced.
There are attractions all around the lake. And if you don't like boats, you can always take the bus! Which doesn't have the same kind of view, but provides an excellent one anyway.
Food around the lake is slightly expensive, tourist price. Bring money!
I had the pleasure of enjoying lunch by the water, before taking a boat to the other side of Lake Chuzenji, and, although the boat was full of noisy schoolchildren, it didn't take away from the experience one bit.
Stunning views of Mount Nantai in the distance, a beautiful lake of clear blue water all around me, and the excitement of being on a boat. An unbeatable experience.