Museum of the City of New York
Now in its second year, the Speakeasy at 1220 Fifth Avenue is a series of Wednesday-night cocktail parties held on a Georgian-revival terrace overlooking Fifth Avenue and Central Park. As one might expect from a museum that specializes in the history of the city, the Speakeasy holds assiduously to a 1920s theme. Music ranges from the Charleston to the blues, and drinks are Prohibition-era specials like manhattans, sidecars, and Bronx cocktails. A handful of regulars even come decked out in Roaring Twenties garb: flapper dresses for the women and knickers, argyle socks, and newsboy caps for the men. Bonus perk: All exhibitions are open during the event, including this summer's hit, "Cars, Culture, and the City." 1220 5th Ave. at 103rd St., mcny.org, admission $15 (includes your first cocktail), drinks from $9, Wednesdays June 30–Aug. 18, 6 p.m.–9 p.m.
The Metropolitan Museum of Art
The Met has been throwing Friday- and Saturday-evening rooftop parties for years, but this season it has raised the bar. To ornament the martini bar in the rooftop garden, which overlooks Central Park, curators brought in "Doug & Mike Starn on the Roof: Big Bambú," an installation that will expand throughout the summer until it is composed of 5,000 bamboo poles lashed together by 50 miles of nylon rope. Take it all in while sipping a Big Bambú ($12), made with vodka, oolong tea, and ginger syrup. 1000 5th Ave. at 82nd St., metmuseum.org, Fridays and Saturdays through Oct. 31, bar open approximately 5:30 p.m.–8:30 p.m., suggested admission $20 (for museum entry).
The Museum of Modern Art
For a museum where Warhol sits in the same room with Brancusi, MoMA's PopRally is a fittingly eclectic party. Held year-round approximately every six weeks, the event has rotating themes (in the past it's been Tim Burton, skateboarding, dance), with music performed live or by a DJ. Guests are encouraged to dress up at some of the PopRallys: In summer '09, many showed up wearing costumes for a screening of The Last Days of Disco. The only downside to the party is that museum access is limited to a couple of relevant exhibits. 11 W. 53rd St., (some events are held at its affiliate P.S.1 in Long Island City, Queens) sign up to receive event notices at moma.org/poprally, dates vary, hours have ranged from 6 p.m. to 1 a.m., admission, $8–15, adults 21 and older only.
American Museum of Natural History
Art is nice, but if partying in a planetarium is more your thing, One Step Beyond is the place. Held within AMNH's Rose Center for Earth and Space, One Step Beyond draws some 1,000 guests who come to drink, dance, and listen to top-flight musicians like Moby and Kanye West. In a separate wing, visitors can take a time-out to catch a space show; the current feature, The Search for Life: Are We Alone?, is narrated by Harrison Ford. On pause this summer, the party resumes one Friday per month during fall, winter, and spring. Central Park West at 79th St., amnh.org, dates vary, hours are typically 6 p.m. to 1 a.m. (check online for updated schedule), admission $25 (includes a voucher for a return visit to the museum), ages 21 and older only.
Cooper-Hewitt, National Design Museum
Sophistication is the name of the game at Cocktails@Cooper-Hewitt, a weekly party in the museum's garden, the largest private green space in Manhattan. Jazz is the preferred soundtrack, views are of Central Park, and this summer's parties are themed to the museum's ongoing National Design Triennial—"Why Design Now?"—open during the event. Expect up to 800 visitors over the course of each night, ranging from Upper East Siders to downtown designers. 2 E. 91st St., cooperhewitt.org, Fridays, July 9–Aug. 13, hours are typically 6 p.m.-9 p.m., admission $15 (for museum entry).