As an urbanist, I’m a little embarassed to admit that I’ve only been in New York’s City Hall twice in my life. The architecture is beautiful.
I think parts of the building, including the rotunda, are open to the public, although the intimidating police checkpoints at both entrance gates seem to suggest otherwise. To enter, you’ll have to put your bags through the x-ray and walk through a metal detector, but you don’t have to explain why you’re visiting. The rotunda alone is more than worth this minor hassle.
Make sure also to get your photo taken on the steps while shouting something and shaking your fist, for the full City Hall experience.