Located in the far north west part of Tokyo, Okutama-machi is one of the most scenic spots. It is difficult to believe that you are still in borders of Tokyo city.
Nippara cave is the biggest cave in the Kanto region and can take about 40 minutes to explore. People might be dissapointed by the lack of any stalactites and stalagmites in the cave, but the area is still an interesting place to explore. Entrance to the cave is 600 yen for adults.
The train trip to Nippara is over 2 hours, followed by a bus trip up the mountain and a final 20 minute walk. The area is also great for hiking.