Mt. Mitsunoge is great hike not too far from Tokyo. The full hike is about 20 KM long and takes about 5.5 hours of walking to finish. I came across this while browsing for things to do around the city on ridgeline images.
I had a spare day remaining on an old seishun 18 ticket (and not much time to finish it) and therefore decided to take most of the route to and from Tokyo by train. I’d recommened going by bus if you’re not in a situtation like I was.
The JR Chuo Line