I have visited this exciting museum in years past, so I knew I would have a hard time seeing all the galleries in one day. Instead I purchased an annual membership on this day with plans to return and take in the place more slowly later. If you're visiting for one day only, I suggest you look at a map online and pick five or six galleries to see and ignore the rest, or plan to walk through them knowing you won't have time to digest them all.
This is without doubt one of the best places to people-watch in New York City. The pigeons will come up close hoping for a crumb or two, and the view is great looking both north and south. There is always a stream of visitors walking by, and it's a common area for the city workers to take their lunch break.
Oh, I should advise you: do NOT feed a single pigeon unless you don't mind their friends and family bombarding you immediately afterwords.