Science buffs should not miss this cavernous museum, Europe's oldest dedicated to invention and technology. It's a treasure trove of wonkiness with 80,000 instruments, machines, and gadgets—including 16th-century astrolabes, Pascal's first mechanical calculator, and film-camera prototypes by the Frères Lumière. You can watch video simulations of groundbreaking architectural achievements, like the cast-iron dome, or see how Jacquard's machine revolutionized cloth-making.
Kids will love the flying machines (among them the first plane to cross the English Channel), and the impressive display of old automobiles in the high-ceilinged chapel of St-Martin-des-Champs. Also in the chapel is a copy of Foucault's Pendulum, which proved to the world in 1851 that the Earth rotated (demonstrations are staged daily at noon and 5). The building, built between the 11th and 13th centuries, was a church and priory. It was confiscated during the Revolution, and after incarnations as a school and a weapons factory, became a museum in 1799. Most displays have information in English, but having an English audioguide (€5) helps. There is a quiet café on the first floor. If you're taking the subway here, check out the platform of métro Line 11 in the Arts and Métiers station—one of the city's most elaborate—made to look like the inside of a machine, complete with rust-color metal walls, giant bolts, and faux gears.