Personally, I would skip going into Tokyo for a layover that short. Leave your luggage at NRT, and take the train to Narita City. If the weather is decent, take a walk to the temple. It's really quite beautiful, and you can spend hours wandering the grounds and taking photos, or relaxing.

Eat some noodles or some sushi, take a trip to the mall if that's your thing, then head back to NRT for a shower at the RCC and a drink before your next flight.

The Mike Newman Narita layover page mentioned up thread has some good info, though some of it may be dated now.