Throughout this summer, if you're in New York, you should stop by one of these two museums on a weekend night since their admission fees go down from about $10-15 to "pay what you wish".
On Friday nights, the Whitney Museum of American Art allows you to pay what you wish from 6 to 9 pm. Right now there are some interesting installations on display: mirror installations by conceptual artist Dan Graham and some pop sculptures by Claes Oldenburg. These are beyond worth seeing any night of the week!
On Saturday nights, the Solomon R. Guggenheim Museum has the "pay what you wish" option from 5:45 to 7:45, and a lot of people already know about it. When I got there this past Saturday, I waited in line on Madison Avenue, about 2 blocks away from the museum's entrance. Since it is the 50th anniversary year, The Guggenheim had an amazing special exhibition of its own architect and designer: Frank Lloyd Wright. It included original drawings and ideas for The Guggenheim's exterior, as well as for urban communities, schools and other institutions.
Exterior of The Guggenheim Museum on 5th Avenue
The exterior of The Whitney Museum on Madison Avenue
Cool street art on Madison Avenue at 75th Street