You can also save money with one of the Suica+NEX deals, especially since your trip is less than two weeks long.

You'll get a roundtrip on the Narita Express to Tokyo and a rechargeable debit card (Suica) preloaded with ¥1500 for public transport (JR and private trains, subways, buses) within the Tokyo-Yokohama area. (It's not good in western Japan, i.e. Kyoto and vicinity, but you can save money on local travel there by traveling on private rail lines instead of JR).

Details are here:

http://www.jreast.co.jp/e/suica-nex/