Hôtel de Ville

http://travel.nationalgeographic.com/travel/city-guides/free-paris-traveler/

In the Marais, Paris's impressive Hôtel de Ville (City Hall, 29 rue de Rivoli, tel. 33 [0]1 42 76 43 43) features a fountain-laden square and free admission. Much of the grandiose building is off-limits for security reasons, but Parisian exhibits and free information are available in the lobby.