Before you decide, do bear in mind Narita is usually about a 90minute journey from central Tokyo, especially Shibuya which is on the West side of central Tokyo. So make sure you leave a good margin of time.
We have an itinerary section:
If you stay on the East side in somewhere like Asakusa, then you the journey time to Narita will be a little shorter and an itinerary like this would make sense:
Otherwise if you stay in Shibuya, though we do have an itinerary for shibuya, apart from visiting Meiji Shrine and Shibuya crossing, if it's your first time I'd say you should probably just wonder around without a plan!
Also we have some ideas if you just stay close to the airport: