I'd like to second Judy's suggestion to visit Narita City and the gorgeous temple there. Local trains leave from the airport very frequently, and you can be in Narita City in 10 or 15 minutes.

When I did this last year I was able to leave my luggage at an airport depository. Arriving in Narita City I then walked from the train station all the way to the temple, a distance of about a mile or mile and a half. The walk is delightful-- along a narrow street lined with charming shops and restaurants. At the end of the street are the temple and garden grounds, which are magnificent.

To me this would be the ideal way to spend 5 or 6 idle hours, close to Narita airport.

Joel, in Chicago, USA