A clear blue lake, a towering medieval castle, and creaking covered bridges—set against a ring of snowcapped Alps. That’s Thun, a small Swiss city between Bern and Interlaken, where you can savor slow travel yet remain close to the alpine attractions that draw millions to central Switzerland.   Like many travelers, I assumed touristy Interlaken was the obvious base for exploring the Bernese Alps last spring.   After a closer look at the map, I noticed Thun (pronounced toon), folded into a corner of crescent-shaped...