I have thought about what to do with the UN building, and have come up with a couple of differing ideas. Besides simply blowing the thing up, you can:

1) Turn it into a museum, along the lines of Auschwitz, highlighting the horrors supported by the institution durings its tenure upon the Earth.

2) Turn it into New York's first legal Brothel. Oh wait...

3)Use it as a naval firing range for ships to use their guns on. After all, since we can't use Porto Rico, why not use something closer to home?

4) Make it into a giant homeless shelter, to help deal with the huge numbers of bums wandering around New York streets. The clientele of the place would improve greatly after such a transformation.