At the end of the business day, escape to one of these exotic happy-hour musts. In Las Vegas, order an atomic-era cocktail at the Venus Lounge & Tiki Bar (702-414-4870) in the Venetian Hotel, which marries a Rat Pack-ish lounge to a tropical bar. Designer Nancy Mah pares the kitsch from the Polynesian formula in the sleek Tiki Room (646-230-1444) in New York’s Flatiron District. And in Chicago, the intimate Whiskey Sky (312-255-4463) puts martini-swilling patrons amid the skyline from its 33rd-floor perch in the W Chicago-Lakeshore Hotel. —