8 hours will be tight to go into Tokyo because Narita is about an hour train ride away from the centre of the city. So, if you're willing to do it, you should get a good 4 hours to explore.
I recommend Shinjuku. Plenty of people watching, plenty of back alley shops and bars. Also plenty of high end shops too. If you want to look the area up on google maps, look for the Hooters in Shinjuku.
From Shinjuku, you can access the Meiji Shrine and you're a one stop train ride away from Harajuku.
Check the times on the JR Japan Rail site for when the Narita Express train runs on the day you'll be there. The NEX train will take you from the airport to Shinjuku station via Tokyo Station.
You'll need to be sure to get some Yen from an ATM at the airport. It will take 30-45 minutes to get through immigration on arrival and maybe 15 minutes on departure.
Spend any unused Yen on Wasabi flavoured Kit-Kats at Duty Free.