Depends on what you want to see of course, or what you're into. If it was me, Id be on the train straight from Narita into Shinjuku and drunk by 3. But you could get into Akihabara (electronic city) within a couple hours. If you head to Terminal 2, you could go to the US Military counter and they will hook you up with a ride into Yokosuka city, even without an ID card..They dont care. This is the Tokyo (JR East) train map, in English, to give you an idea on where your at. Narita is southeast of Tok-yo proper, but easily accessible.
Or just hitch a ride with someone anywhere. Sound like Im crazy? Its not the US - I did it ALL the time..
All you'll find that someone will be MORE than happy to give you a ride, since they can practice their English on you...Take pics!!!
http://www.jreast.co.jp/e/info/map_a4ol.pdf