One thing to remember is that Narita Airport isn't really in Tokyo.
From my experience, it takes nearly an hour to clear immigration and customs, thats after deboarding (which is a lengthy process itself).
Then by fastest train (either skyliner, or Narita Express) it takes about 1hour to get into Central Tokyo from Narita Airport and can cost between $40USD-$60usd roundtrip. NEX runs about every 30minutes, so you may/maynot have to wait about 30minutes for the next trains.
Then on your return trip from Tokyo 1 hr, reclear about 2-3 security checkpoints, and clear immigration/passport control, In reality you have about 1 hour to see central tokyo, not really enough time to do much (as it can take 15minutes to 1 hour just to get around Central Tokyo. There are a lot of factors the come into play timing wise, so that visiting Tokyo isn't really feasable.
You can however try to visit nearby narita city, its a nice town with various shops and temple, more info can be found at the Narita Layover page:
www.mgnewman.com/narita/
I hope that info helps your planning.