Hi

How long is your lay-over in NRT ?
What class are you travelling ?

There is no need to go through immigration nor customs inspections as long as your checked luggage has been correctly tagged to GUM.

If you have more than 4 hours and you want to go through the trouble of immigration, you could do a quick visit to the Narita city and its famous temple.

It is possible to go to terminal 1 but it means taking a bus (still no need to clear immigration nor customs) there and back.

If you are flying business class CO will give you access to one of the 7yamato' lounges at NRT terminal 2 (I am not sure about showers in the lounge)
Showers are available for a nominal fee in terminal 2 but you might get a voucher from the Yamato lounge.
There is a people mover between satellite C/D and A/B (where most of the shops/eating places are)