For someone unfamiliar with Japan, a trip to the town of Narita may be a bit ambitious, since you can never predict how long it will take to go through immigration and customs and then check back in.
However, the shopping area in the airport is a pretty good place to kill a couple of hours, with lots of souvenir shops (and not all the souvenirs are junky--some are quite nice), a bookstore, an electronics and music store, stalls that sell boxes of the types of foods that appeal mainly to Japanese people (I imagine that people take them to their homesick relatives posted overseas), and several restaurants and snack bars serving a wide range of food and drink both Japanese and international.