From the Mixed-up Files of Mrs. Basil E. Frankweiler by E.L. Konigsberg (1967)

When I went to New York for the first time, one of the first things I wanted to see was the Metropolitan Museum of Art.  Why?  That's where Claudia and Jamie ran away to in one of my very favorite books as a kid (with one of the longest titles).  Fed up with her life and chores, Claudia plans an elaborate scheme to run away to New York City, and stay in the Metropolitan Museum of Art.  But being by Konigsberg, that's not all.  They also get immersed in a mystery involving a statue which may or may not be by Michaelangelo.  But for me, it's the little details that stay with me like fishing for coins in the fountain (then using them at the Automat!), and hiding in the restrooms at closing.  Other beloved Konigsberg books include The View From Saturday and Jennifer, Hecate, Macbeth, William McKinley and Me, Elizabeth.