I'd have to agree that the Met is the must see, but must see depends on what interests you.

If it's history or science, then the Natural Museum of History, if you're a book person, then the NYPL, etc...

I'm less than a 10 minute walk from the Met and have been going there about once a week for years. I'd go to the web site and pick 2 or 3 favorite sections and get familiar with the layout. That will maximize your time spent.