[halfnihonjin]

I would definitely go to Shibuya and go to see the crossing there if you havent already. The Starbucks across the street from the station has a great view from the second story of all the people walking across. If you are more into temples and whatnot I would go down to Kamakura, it is a little far down to the south though. Harajuku is always interesting just to see the fashion of the people who shop there.

Coming out of the Yotsuya station there is a tiny street to the right of Shinjuku Dori that is full of hole in the wall bars and restaurants. I studied at the university at the station and explored quite a few of them. The Japanese who go there were always up for conversation too.