At home in the New York City suburbs, Oscar Sandoval has lots of friends and an active social life. But when it’s time for vacation, he prefers to keep it quiet – literally. Sandoval began practicing Zen Buddhism a few years back, and has been on silent retreats to Buddhist monasteries around San Francisco and elsewhere. He’d stroll, sit, do some gardening and generally contemplate life for a week. More recently, he’s done solo backpacking trips across Spain. “The internal experience varies from times of very little thinking to periods of many thoughts or songs playing in my head,” he says. “T…