Subject: Not the big house
When we hit a city for the first time the "Big House" museum tops the list of what's most important to see. Whether it's the Louvre, the Hermitage, the Metropolitan, the Smithsonian, Prado or whatever, you are in for a building which would take days to see and for which you need a map and good shoes just to see the "good stuff".

For those who want to see the highest density of "good stuff" (16th-19th century European old masters) in a small package, I would highly recommend NYC's "Frick Collection (10 E. 71 Street, corner of 5th Avenue, https://www.frick.org/). This moderate sized museum packs more Rembrandts, Holbein, Vermeer, El Greco, Van Dyke, Valesquez, etc. etc. into a space able to be seen in 2-3 hours than you can find scattered through most of its lager competition. Of course, if you haven't seen enough when you've "finished" the building, there are roughly a dozen other top museums withing a mile radius - the Metropolitan, Museum of Modern Art, Cooper Hewitt (one of my favorites), Guggenheim, Neue Galerie (another favorite of mine) pop to mind, but there are others.

Jeff