You will not regret choosing Japan as your travel destination!
If you are alone, check out couchsurfing - you could stay for free at people's houses and make friends.
And don't worry, a couple nights out in Tokyo (Shibuya, Roppongi areas) and you will meet people in no time. Try some foreign bars like Hub's.
If you like history, Hiroshima is a good choice, as well as Nara and Kyoto.
Although 2 weeks in Tokyo will not bore you!! I usually recommend at least 3 weeks there.
There are some useful tips on my blog which you can check out -
Smart & Cheap Japan Travel. The book that is coming out shows how to travel in Japan the right way and do so economically.