A cute tourist area tucked away in Yamanashi Prefecture is Lake Yamanaka. A bright blue lake with a stunning backdrop of Mt. Fuji, it is about a 2-3 hour trip from Tokyo.

You can take the Kaiji Express train directly from Shinjuku Station to Otsuki, then take the Fujikyu Line (or express train) from Otsuki to Fujisan, and finally take a bus (about 30 minutes) from there to Yamanaka. It's worth the day trip, but if you want, you can make it a few-days trip, as there are plenty of hotels, hostels and camping grounds near the lake.