definitely onsen, if your comfortable with the bathing culture. it feels great when you go there in january! if you are planning to go to famous shrines then everything will be fine. only small shrines in particular are SOMETIMES unsafe at night. if you go there whrn the sun is out, it would be very nice. hiking, well, if you are not talking about like hiking mt. fuji or something then you will be fine. i recommend mt. takao. its a nice little hike and very accessible from central tokyo!
@toko_jp_eng thanks a lot!! do you know by any chance places where I can practice Japanese? like places specifically made for language exchange? I'm going for a month so I was thinking that visiting a place like that regularly might be a good idea to get to know people and such