In New York, you can always visit the Guggenheim in Soho for free, as well as The Whitney at Phillip Morris and The Museum of American Folk Art. A number of other museums offer a "suggested donation" admission, and are instructed to treat you the same whether you're donating $1 or the full fare. Here's a pretty good list for New York.
The Art Institute of Chicago is free from 5-8 on Thursdays, Los Angeles free days are listed here, and if you find yourself in Paris, the Louvre is free on the first Sunday of every month and on Bastille Day, July 14th.
Be sure to Google museums in your area, or check the website of any museum you intend to visit to see if there's a free day of which you can take advantage.
It's just another easy way to make life more lux for cheap or free.