La Place de la Bastille
A lot of foreign visitors to Paris think that the infamous Bastille prison is still standing, and they want to go find it and take a tour! Then I've gotta break it to them that the prison was destroyed in 1789 during the French Revolution; you know, they, like, stormed it. Don't worry, even Jack Kerouac owns up to making the same mistake when visiting Paris in Lonesome Traveler (1960), and I sure as heck had never even thought about it one way or the other when I first came to Paris on vacation many years ago.
Nowadays the spot where the Bastille used to stand is marked by a big monument called Le Colonne de Juillet (the July Column) in the middle of a traffic circle known as the Place de la Bastille. There's not really much to do in the area other than look at the monument for a minute and then maybe eat at a touristy restaurant on the edge of the Place. But it's still worth checking out momentarily just to say you've been to the spot where the Marquis de Sade was imprisoned for ten years.
While you're in the neighborhood, if you feel like exploring a bit you can head up the Rue de la Roquette, one of the smaller streets that radiates out from the Place, and have a drink at Les Furieux or La Fée Verte, depending on whether you're in the mood for beer and rock n' roll or absinthe and quaint old-fashioned Paris.