Two things I try to be aware of when I travel. Are you allowed to take photos in the museums? (Mostly yes, without flash; some museums, like those in Cairo, charge you to bring a camera inside.) When are the museums free? (Varies. Madrid’s Prado is free to all most evenings and Sunday; Paris’s Musée d’Orsay has a more complicated policy, of course. Check local listings.) Admission to Boston’s Museum of Fine Arts on Wednesdays from 4pm to 9:45pm is by “voluntary contribution,” which for many means “free.” Such MFA freedom allows an hour or so to look at one or two things without the burden of the normal $22 admission fee. For example, on this afternoon I sampled the “Avedon: Fashion, 1944-2000” exhibit (Suzy Parker, Dovima, China Machado, Jean Shrimpton, Penelope Tree, Twiggy), a show of Japanese tattoo paintings, a moment with the Eqyptian mummies, a walk through the Catalonian chapel. One hour and fifteen minutes of visual delight, accompanied by the pleasure of eavesdropping on many foreign visitors speaking a lovely mix of Italian, Spanish, French, Portuguese. Gratis.