We went for our honey moon and there are plenty of options.
How about Grand Central Station (very impressive building and you will always remember your day when you see it again)
“The Grand Central Whispering Gallery
The “whispering gallery” is located on the Grand Central Terminal dining concourse near the famous Oyster Bar & Restaurant. Here, the acoustics of the low ceramic arches can cause a whisper to sound like a shout. Sound impossible? To test it out, you and a friend will have to stand in opposite corners of the large arched entryway. Now face the corner and whisper. Your friend should be able to hear your voice as if you were right next to them, not whispering into a far-away corner.
According to experts, this happens because the whisperer’s voice follows the curve of the domed ceiling. The Whispering Gallery is a popular spot for marriage proposals – and a unique place to whisper sweet nothings to your main squeeze.”
And then off for food in the Oyster bar. May not be very private and personal if you like that sort of thing, but hey, it NYC.