You'd have a great time but you'd need way more than a day. My girlfriend had been to NYC before, but never lived here when she moved here in '05. I've taken her on many tours of old architecture since, fortunately, she loves it as much as I do. By now, of course, she's seen all the major ones, but less-well-known ones that are still impressive still pop up here and there for both of us. There are - no exaggeration - hundreds of pre-war buildings in this city to see, literally, hundreds. And a meaningful number of them (heck, most of them) would be the most impressive building in the town I grew up in, in Central Jersey.