Honestly though it would be a tragedy if you just spent all 8 days in Tokyo. About 3-4 days are enough - there are some very good day trips to take in the area, and Japan is not all one big Tokyo. Some good places are Nikko, Hakone, Kamakura, the Fuji 5 Lakes, Izu, Kusatsu, Kawagoe, Takao, and Karuizawa. A Tokyo Wide Pass is a great way to see them and save money: http://www.japan-guide.com/e/e2361_06.html For the Fuji area there are good Odakyu passes: http://www.odakyu.jp/english/deels/
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If you're only going to be in Tokyo, then neither a regional pass nor a JR Pass makes sense.
You can use a 3 day subway day pass (or 2) to get around and save money.
http://www.tokyometro.jp/en/ticket/value/travel/index.html#anc03
Honestly though it would be a tragedy if you just spent all 8 days in Tokyo. About 3-4 days are enough - there are some very good day trips to take in the area, and Japan is not all one big Tokyo. Some good places are Nikko, Hakone, Kamakura, the Fuji 5 Lakes, Izu, Kusatsu, Kawagoe, Takao, and Karuizawa. A Tokyo Wide Pass is a great way to see them and save money:
http://www.japan-guide.com/e/e2361_06.html
For the Fuji area there are good Odakyu passes:
http://www.odakyu.jp/english/deels/