It's a bit strange that Mike Newman's site was unavailable, but it seems to be back up now. The link is http://net.saipan.com/personal/mike_...ta/narita.html

Seven hours is enough to get into Narita and back, although you'll have to wait in possibly 4 lines (immigration, customs, check-in, and departure immigration). It's best if you're elite on the carrier you're flying and don't need to wait in a long check-in line. No one takes dollars; get Yen. You can change any excess back again on your return to the airport.

If you're dead tired, you can take a nap in a day room at the airport instead. If not, be adventurous, explore!!