Kawaguchiko Music Forest [ 河口湖オルゴールの森 ] is a museum-type theme park on the northeast side of Kawaguchi Lake that gives you a view of Mount Fuji on a clear day. There are 3 exhibition halls with organs and music boxes on display as well as a fountain and a cafe/restaurant with shows outside.
There are shows every hour in each exhibition hall. I didn't find it that amazing, but it was an okay way to kill a couple of hours, especially if the weather isn't good. The main concert hall in the 3rd Exhibition Hall was quite cool, with pastel-colored dolls coming to live banging on musical instruments playing songs. It was weird but interesting.
It costs 1300 yen to get it, but sometimes there might be 100 yen off coupons on the street outside. They'll give you an English map if you need one. Kawaguchiko Music Forest or Japan Guide will have more information.