It depends on what you want to see in Japan.
There are many attractions just in Tokyo and around Tokyo. If you are willing to take a day trip from Tokyo, there are attractions such as Kamakura, Nikko, Kawagoe, Hakone, Mt. Fuji, etc.
In Tokyo, there are lots of museums, shopping malls, bars, Izakayas. Great food gather in Tokyo but there are many local cuisines you can enjoy in different parts of Japan.
If you have just a week, I'd say stay around Tokyo and maybe take one or two day trips. There are so many attractions in Tokyo and it won't be enough if you just have two or three days there. (Although most tourist stay only for 2-3 days in Tokyo and Kyoto)