Lake Kawaguchi [ 河口湖 ] is one of the Fuji Five Lakes [ 富士五湖 ] where you can see Mount Fuji [ 富士山 ] when the weather's good. It's a quiet lake where people go boating and fishing. There are lots of hot spring onsen [ 温泉 ] hotels as well as museums and attractions in Kawaguchiko.
How many days in Kawaguchiko?

Being only 2 hours by bus, it makes a nice day trip from Tokyo as there's not much going on there at night. We stayed overnight and just took it easy walking around the lake, enjoying the autumn colors. The lake is less than a 10-minute walk from Kawaguchiko Station [ 河口湖駅 ] where trains and buses stop. There are picnic tables near the lake so you can buy food from the convini and eat by the lake. During the Autumn Leaves Festival from November 1st to 30th, there are food stands at night and lit up maple trees on the northeastern shore near the Kawaguchiko Music Forest. We also hiked up the Kachi Kachi Ropeway which gives you a nice view of Mount Fuji on one side and Lake Kawaguchi on the other.
If you don't feel like walking, there's a bus once an hour that goes around the lake and also takes you to a smaller lake named Saiko [ 西湖 ]. It stops running around 6 at night though.
How to get to Kawaguchiko?

You can get to Lake Kawaguchiko by train, but we took the high speed bus that costs 1700 yen and 2 hours each way. There are 1 or 2 buses per hour. You can book the tickets online at ハイウェイバス to get a reservation number then use machines at convenience stores to pay then get tickets with assigned seats.
The buses departs from Shinjuku High-speed Bus Terminal [ 新宿高速バスターミナル ] that's across the street from Keio Department Store Shinjuku Station's [ 新宿駅 ] West Exit.
You can find more information on what to see on Japan Guide : Mount Fuji : Yamanashi : Fuji Five Lakes : Lake Kawaguchiko