Mt. Takao is probably the easiest option if you want to go mountain hiking in Tokyo. Many elementary schools send their students there for day trips, which reflects that it’s a very easy hike, even for children! The easiest course only takes about two hours to get to the top. If you are still worried it might be too tough, you can ride a cable car to the middle of the mountain. At the base of the mountain is a popular tourist area with various shops where you can enjoy a variety of food and even a soak in the hot springs! Mt. Takao is only an hour and a half away from central Tokyo.
(Source: Japan Travel by NAVITIME)