It seems like every time of year is "in season" in Lenox, a small Berkshire town that has been a popular retreat since the 19th century. In summer, picnic on the lawn at Tanglewood, the Boston Symphony Orchestra's summer home. In fall, when foliage peaks, take to the hiking and biking paths of the Pleasant Valley Sanctuary to immerse yourself in the vibrant colors of the mountains.  Winter visitors enjoy skiing at a number of resorts and cross-country trails, and in spring, hikers and golfers practice their pastimes in splendid settings. Rejuvenating treatments and pampering, at the well-known Cranwell Spa, are favored all year long.