The country is opening more and more every year. Most things that were off-limits a few years ago can now be visited, including the entirely length of the Pyongyang metro, and almost every major city.
One of the more interesting places that's still off-limits last I checked is the DPRK parliament building:
http://www.uer.ca/...=1&threadid=119594 Ask nicely and maybe they'll let you in.
You used to be able to get into the base of Ryugyong hotel through the needleworks factory shop thing, but that was before they resumed construction.
A not terribly well known 'secret' is that you can catch taxis to get around Pyongyang on your own. There are foreigners living and working there, and the drivers can't tell the difference. I wouldn't recommend walking around.
There is tonnes of stuff to see without getting into dodgy territory. I'd highly recommend going if you get a chance.