"You probably think you're mentally on top of your game," says Jack Kornfield, a 68-year-old former Buddhist monk who now advises the U.S. Marines on mental training. "But email, cellphones, TV, your job, your family – all these things are messing with your neural circuitry and hindering your potential." Members of the Marine Corps aren't the only acolytes lining up behind gurus like Kornfield. Meditation retreats have become mainstream destinations as devoted to cleansing the mind as they are to perfecting the body, all amid high-end spa settings. In addition to unplugging and de-cluttering your brain, these seven retreats – from Big Sur's cliffside hot springs to a Hawaiian silent sanctuary to boardroom-boosting mental exercises in the Berkshires – round out their spiritual curricula with adventure activities like hiking, skiing, and kayaking. "To be a really strong person, you have to make time for inner development," Kornfield says. "You have to work out the mind as hard as you work out the body. That doesn't happen on its own."