Great thread idea!
Mt. Takao is about 1 hour from Shinjuku Station in Tokyo on the Keio Line. Kind of a pain the find the platforms in the station, but other than that it's a straight forward trip. There is a Limited Express that requires an extra fare (so you can't just tap on and off with a SUICA card) but it's a time saver to take that instead of the local train.
Once you get there you have the option of either walking all the way up (about 2 hours) or taking a chair lift or inclined railway halfway up and walking the rest of the way.
The path to the top if a typical Japan temple mountain walk, with well paved paths and stairs, lots of shrines and temples, and most of the year, lots, and lots of people. I try to get there when the chairlift and incline railway first open (8am or 9am depending on the time of year) to beat the crowds.
From the top you can get a great view of Fuji-san, and there are various trails that continue on along the ridgeline. There are a bunch of food stalls along the way along with a Monkey Park and some gift shops.
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Due to a quirk in the train schedules, coming back in to Tokyo (if you are going to the eastern half of the city), it's faster to transfer to the Chuo Line after one station.
Our HND trips get in between 5pm and 10pm, and then go back out the next night. On most trips (unless there are other things I need to do in Tokyo) I'll wake up early, head out to Mt. Takao, and then be back in the city by 1pm to get a nap in before heading back to work.