Yamanashi Gem Museum is an excellent place to spend a rainy day. Here, adults and kids alike can learn about all things science and have a bunch of fun at the same time.
You'll have no end of sightseeing choices when in Fujikawaguchiko. Yamanashi Gem Museum is a good place to start here, but there are plenty of other fine spots to check out. Lake Kawaguchi, Fuji-Q Highland, and Fujiten Snow Resort are not far from the display of scientific achievements and will round out your day nicely.