We did the Empire State Building at night and then walked up and got pizza at joes, checked out time square, grand central station, etc. all of that was just walking distance but I’d take the subway to for the experience
The next day we went out to the Statue of Liberty and did the 9/11 museum. I’d really recommend doing that, it was really awesome. Buy tickets ahead of time if you can so you don’t have to wait in line.