Be a cheap ass -- don't rent a hotel if you're stay there for more then a week. Either youth hostel (you can bang foreign chicks) or capsule ... or INTERNET CAFE. You can sleep on the chair.
The youth hostels suck. They have a 10:00 curfew or something rediculous. And in Japan rules are not made to be broken, so you come back at 10:05 and you have a problem. I don't know your budget, but there are "business hotels" that are kind of basic hotels that aren't
too expensive, but aren't lame like the hostels.
I live in Osaka so I don't know the Tokyo details that well. I stay with friends when I go there. But if nightlife is your thing then you want to be near either Roppongi or Shibuya. Roppongi's a bit sleezy but it's the easiest place for gaijin (us!) to pull girls. Shibuya's a bit tougher but the potential reward is hotter.
If you want to learn about the girls you can check out Japanlair.com (Japan PU site) and have a read. Freaky shit goes down here on a regular basis.
In terms of sightseeing in Tokyo itself it's really just checking out the flavor of the different neighbourhoods. Akihabara for electronics and otaku stuff, Shibuya for youth culture, Harajuku is kind of counter culture, Asakusa has more of an old Japan flavor, all that. Sensoji temple in Asakusa is cool.
A little bit out of Tokyo (under an hour I think) is Kamakura, which has a lot of cultural/historical stuff. Also Nikko is another historical place with old temples and stuff (and monkeys). It's a couple hours north of Tokyo.
You gotta try kaiten-zushi, those are the sushi bars where you choose plates from a revolving conveyor belt. Also yakiniku is Korean bbq, but popular here. If there's any single meal on earth made for men, that's it.
Let me know if you come out to the Osaka area.