On Saturday and Sunday, hundreds of the city’s most guarded architectural gems — from private residences to historic landmarks — will unlock their doors as part of Open House New York. Now in its 17th year, OHNY is bigger than ever, with an unprecedented number of sites offering free tours across the five boroughs, providing a glimpse into the way design shapes, defines and propels life in the Big Apple. Don’t know what to see? Here are our picks.

City Hall

One of the oldest City Halls in the nation, this 1812 building just went through its first renovation in 50 years. The glorious rotunda, with central oculus and “floating” marble staircase, is the main attraction, but visitors can peep the City Council Chamber and Governor’s Room, too. (Entrance gate on the east side of Broadway at Murray Street. Open Saturday 12 p.m.-4 p.m.)

Four Seasons Restaurant

For once you won’t need a reservation to enter the world’s fanciest dining establishment. Marvel at the modernist design, shimmering chain curtains and famed Pool Room, site of many a debauched — sometimes skinny-dipping-involved — evening. (99 E 52nd St. Open Saturday 12 p.m.-4 p.m.)

Kings Theatre

This 1920s movie palace, inspired by Versailles, had been languishing in disrepair before undergoing an extensive restoration and reopening earlier this year. Ooh la la. (1027 Flatbush Ave, Brooklyn. Open Sunday 1 p.m.-5 p.m.)

TWA Flight Center

Currently undergoing a restoration, this disused terminal remains a modernist, sleek tribute to the Jet Age, so visit it before it becomes an airport hotel and museum. (Terminal 5, JFK Airport. Open Sunday 11 a.m. to 3 p.m.)

National Arts Club

This private clubhouse, overlooking tony Gramercy Park, not only boasts some incredible art and museum-worthy period furniture; it also has one of New York City's most stunning — and secret — bars, with a dazzling 19th century stained glass ceiling. (15 Gramercy Park South. Open Saturday 12 p.m. to 4 p.m.)

New York State Pavilion

Get up close and personal with Queens’ quirkiest structure, built for the World’s Fair and currently in the process of a restoration. A rare opportunity; expect long lines. (Flushing Meadows and Corona Park, Queens. Open Saturday and Sunday 1 p.m.-5 p.m.)

Sunset Park Material Recovery Facility

See how this site processes the bulk of NYC’s metal, glass and plastic recyclables, in spectacular, green fashion. The building, located on an 11-acre waterfront pier in Brooklyn’s Sunset Park, isn’t too shabby either, winning design awards and boasting some incredible views of the Manhattan skyline. (472 Second Ave, Brooklyn. Open Saturday 2 p.m. - 6 p.m.)