JR Access

Hi,
A 7 day pass would work well for you. A 14 day pass would not pay off.
You can ride JR trains as far and as often as you like within validity. The only train you need to be concerned about is the Nozomi bullet train, which is the fastest, and you cannot board it. But you can still travel very quickly from one part of the country to another.
What you can do is use it from the 16th to go to Okayama and back. Also since you still have some time left with the pass in Tokyo, you could do a day trip to Nikko on the 22nd if you like. (Or activate your pass on the 15th, go to Nikko, then from the 16th to Okayama and back by the 21st).
After that, you could use a 3 day subway pass if you wish:

http://www.tokyometro.jp/en/ticket/value/travel/index.html#anc03

To get to/from Narita cheaply, you can also use a cheap bus for about 1000 yen:

http://accessnarita.jp/en/home/