After a long day of fashion shows, what’s left to do but make the party rounds? Along the rue Saint-Honoré, the city’s hub of international luxury brands, there’s always something new to celebrate

On Wednesday night, white-shirt maven Anne Fontaine hosted a party at her boutique (370 rue Saint-Honoré) to introduce her new day spa. To enter, guests walk into the shop and downstairs, past a floor-to-ceiling wall of trickling water. Outfitted with massage rooms, sauna, and hot tub, this is one spa worth seeing for the ultra modern, compact design alone. A signature massage was developed for the spa and is delivered on tables made of a dense foam material that gives just enough to be comfortable. Above the tables, tiny jets allow for a mist of water to be released over you, if you wish. The square hot tub is designed to overflow like sake out of a wooden cup, and a king-size bathtub is available for couples wishing to have massages followed by a private bath. And in an attempt to keep the white floors as pristine as possible, guests were required to cover their shoes with the same elastic booties used in hospitals

Meanwhile, Spanish designer Josep Font was holding a small gathering at his shop (11 rue de la Sourdière), just off rue Saint-Honoré near the design-centric fashion boutique Colette. Though Mr. Font has four shops in Spain and one in Paris, he is little known in the American market. His clothes emphasize creative details, prints of his own design, and whimsical use of color. Next year, he will present his first haute couture collection, a milestone worth celebrating. His sales team is hoping to use that momentum to bring more attention to the ready-to-wear line, and to bring that line to New York retailers.

Farther down the lane, Costume National (5 rue Cambon) — on a street is famous for Chanel’s headquarters and retail boutique — was celebrating its 21st year in business. It may be an unusual number to celebrate, but the Italian brothers who launched the company have found several numerical coincidences that make “21” special for them. Designer Ennio Capasa, who presented his spring collection just hours before, signed copies of the new Costume National retrospective book published by Assouline. The hipper-than-anyone guests came dressed to celebrate this sleek, fashion-forward brand.

On Tuesday evening, a well-heeled crowd squeezed into the slim boutique of the outerwear brand Moncler (5 rue du Faubourg Saint-Honoré), a credit card’s throw from Roger Vivier. Founded in France more than 50 years ago, Moncler is on a push to establish more free-standing stops, and this is the brand’s first boutique outside of a ski resort. In a nod to Moncler’s wintry heritage, the drinks on offer included warmed red wine with cinnamon sticks.