If you're staying and only sightseeing around Tokyo, I suggest the one of the two discount passes:
KEISEI SKYLINER & METRO PASS 2-days:
Tokyo Metro Two-day Open Ticket (Adult) + Keisei Skyliner Round Trip Ticket (Limited Express): Cost: 4,880 yen
Where to buy: Overseas travel agencies (Korea: TOURBAKSA, Taiwan: LION TRAVEL, Hong Kong: JAL Satellite Travel) or onboard Peach Aviation domestic flights bound for Narita Airport
Where to exchange: Ticket counters at Narita Airport Station or Airport Terminal 2 Station, Keisei Skyliner ticket counters at Keisei Ueno Station or Nippori Station (4 counters) (Seat reservations for Keisei Skyliner are only available if there are seats available)
This pass covers:
[Keisei Line] Round trip from Narita Airport Station or Airport Terminal 2 Station to Keisei Ueno Station
[Tokyo Subway Ticket] All Tokyo Metro Subway lines over 2 consecutive days
--OR--
Keisei Skyliner & Tokyo Subway Ticket
Tokyo Subway 2-Day Ticket + Keisei Skyliner Round Trip Ticket (Limited Express):
Cost: 5,100 yen
Where to buy: Ticket counters at Narita Airport Station or Airport Terminal 2 Station
This pass covers:
[Keisei Line] Round trip from Narita Airport Station or Airport Terminal 2 Station to Keisei Ueno Station
[Tokyo Subway Ticket] All Tokyo Metro and Toei Subway lines over 2 consecutive days
I recommend the Keisei Skyliner & Tokyo 2-Day Subway Ticket between the two passes because the Tokyo Subway Ticket covers both Tokyo Metro and Toei Subway lines (2 of the 3 train lines in Tokyo) and you can buy it at Narita Airport when you arrive (vs buying it overseas). Remember, that this pass DO NOT COVER any JR trains.
see website for more information:
http://www.tokyometro.jp/en/ticket/value/airport_bus/index.html#anc02Hope this helps!