Narita City is a perfectly nice little town to look around.

However, it's worth noting that you can get to Tokyo's Ueno station in a little over 40 minutes by Skyliner train. And the Narita Express to Tokyo station is a one hour trip.

I highly recommend that you look for more ideas about the NRT stopover in the Sticky thread at the top of this forum. The only caveat is that trains are operating reduced schedules since the 3.11 earthquake that struck the North East of the country.