Travelling in Asia, Where is best?

I'm planning on doing some travelling in NE Asia next summer, but on limited time and money I was wondering what the best places to see are. I'll be based in Shanghai from May-October, and I'd like to visit Tokyo, Seoul and Hong Kong, possibly Beijing and P'yongyang if my budget stretches that far. For anyone who's visited these cities already, which are the best for overall experience and culture? And if anyone at all has been to North Korea some tips would be great, thanks!