Hôtel de Ville
Time Out says
Rebuilt by Ballu after the Commune, the palatial, multi-purpose Hôtel de Ville is the heart of the city administration, and a place in which to entertain visiting dignitaries. Free exhibitions are held in the Salon d'Accueil (open 10am-6pm Mon-Fri). The rest of the building, accessible by weekly guided tours (book in advance), has parquet floors, marble statues, crystal chandeliers and painted ceilings.
In summer the stretch of Seine nearest to the Hôtel de Ville becomes Paris' city beach, "Paris Plages", while in winter, the square in front of the monument is used for ice-skating.