I had the privilege of visiting New York for the first time and, being a museum lover, made a goal of seeing the three major museums of art.

Metropolitan Museum of Art
1000 Fifth Avenue, New York, New York 10028
Phone: (212) 535-7710
Website: www.metmuseum.org

Solomon R Guggenheim Museum
1071 5th Ave., New York, NY 10128
Phone: (212) 423-3500
Website: www.guggenheim.org

Museum of Modern Art
11 W 53rd St, New York, NY 10019-5497
between Fifth and Sixth avenues
Phone: (212) 708-9400
Website: www.moma.org

I must say, all three of these museums were incredible.
For a day trip, I suggest the Metropolitan Museum of Art. I toured it in 5 hours and was able to explore each and every room (many on accident because I got lost quite a few times). I did this visit without a break and I must admit, I was extremely exhausted after. I also visited first thing in the morning when it wasn't so crowded. By the time I left, the museum was extremely busy. So, for most, a day trip for this museum is reccommended.

The Guggenheim and the MOMA museums, respectfully, could be a single day trip for both.