It's a great day, the sun is shining, it's not too cold, so how about a day of hiking in Tokyo?
Spreading out a map of the megacity, there's not much in the way of green areas, and there are even fewer places with yama (mountain) or oka (hill) indicated.
But hold on, what's this big blob of green on the west side of the metropolis? Some place called Takao-san. It looks as good a bet as any, and it's easy to get to; at the end of the Keio train line, it's just an hour by train from Shinjuku Station.