In summer, the Norwegian capital basks in about 21 hours of sunlight daily and residents seem to enjoy every one. Their favorite new playground for a leisurely Sunday is Tjuvholmen, a promontory along the Oslo fiord where thieves hid out in the 18th century. That reputation only adds to the hip allure of this newly developed district of waterfront walkways, shops, cafes, and a high-design hotel called The Thief — only a short walk from the city center. The architectural standout is the Astrup Fearnley Museum, which displays provocative contemporary art. The complex of angular wood, glass, and metal buildings was designed by Renzo Piano, who leaped to fame in the ‘70s after collaborating on the Centre Pompidou in Paris, and recently designed the new wing of the Isabella Stewart Gardner Museum and the reboot of the Harvard Art Museums. But Paris and Boston got only great buildings: Oslo got a great building on the beach. The sand is a bit rocky on the pocket beach in the shadow of the museum. But that doesn’t deter sunbathers from stretching out on towels as little kids play in the shallows, teenagers flirt along the seawall, and the heartiest take a swim in the sun-warmed waters.