Tango Contemporary Cafe   In a city that begs for a cafe society — it's either Alan Wong's or a plate lunch — where's the middle ground? It's at Tango Contemporary Cafe, the baby of former Hawaii Prince Hotel staffers Goran Streng and Tami Orozco. The duo has adopted the winning formula of high design and low prices. The best thing on the menu may be the textbook gravlax. The silky, moist, salted and housecured salmon (a dish shared by Swedes and Finns), paired with dill-mustard sauce, is amazing. Streng's "less-is-better" approach rings true in the mustard-herbcrusted rack of lamb, the salty coating playing up the faint gaminess with simple directness. With an inviting space, amiable service and affordable cafe food, Tango Contemporary Cafeis one of the best new things this city has to offer.  1288 Ala Moana, Hokua building, 593-7288. Breakfast: 7-10 Mondays Fridays, 8-10:30 a.m. Saturdays Sundays. Lunch: 11 a.m.-4:30 p.m. daily. Dinner: 5-9:30 p.m. daily. AX, DS, JCB, MC, VS. No checks. $$.  Web site: http://www.tangocafehawaii.com/