One of the most popular spots to see red leaves in Japan is the Maples Corridor or "Momiji Kairou" at Lake Kawaguchi. Momiji means red leaves and a "Momiji Matsuri" festival is held every year from the end of October to mid November with road stalls and romantic night illumination, so it is a popular destination during November for people to see both Mount Fuji and red leaves.
Getting there from Tokyo
There are a few ways to get to Kawaguchi-ko Station from Shinjuku Station. Odakyu offers a 3-day Fuji Hakone Pass for ¥7,400 and JR offers a 2-day round trip ticket for ¥5,600. The JR round trip ticket includes a limited express train called Kaiji 101 that you can board on a JR platform at Shinjuku Station. You can also ride the JR Yamanote Line with this ticket. However, if you are not going up to Mount Fuji's 5th Station but you are just planning to go around Lake Kawaguchi and you are already at Shinjuku Station, I recommend to buy separate tickets at the counter that will cost around ¥2,500 one-way. You can also take local trains, but that will take around 3 hours for just one-way and costs around ¥2,000. Highway buses are cheaper at ¥1,750 and it takes 2 hours; however, tickets get sold out pretty fast.
Most trains will not go directly to Kawaguchi-ko Station so you will need to transfer to a Fuji-san Limited Express train on the Fujikyuko Line at Otsuki Station. It will take around 50 minutes to get to Kawaguchi-ko Station. If you are there early and plan to visit at least 2-3 attractions, then buying a 2-day bus pass for ¥1,200 is worth it. You can hop on these buses to get around the lake easily. There are two main bus routes: the Green-line that goes towards the west of the lake, and a Red-line that goes to the east of the lake where the red leaves are and will cost ¥380 if you did not buy the pass. If you did not buy the pass, you will have to pay exact cash when you get off the bus. The bus operates daily from 9-6pm so staying too late is not possible.
Best Time to View the Maples
The best time is usually from beginning to mid-November. If you want to catch Mt Fuji, the best time is actually early morning like 6-7am before the clouds roll in. Therefore the best way is to stay overnight at a lodging there and waking up early to catch both the red leaves and a fantastic Mt Fuji view. Skies become dark by 5pm so you can catch the red leaves night illumination for a while before you return.
Other Tips for Maple Viewing
It can get chilly at night so be sure to bring enough warm clothing. I definitely recommend staying at Lake Kawaguchi for 2-3 days (including going to Gotemba Premium Outlets or Fuji-Q Highland amusement park) to explore to its fullest and enjoy the beautiful scenery.