Historically, Portugal has been much farther down on visitors’ lists than France, Italy and Spain. But more and more travelers have discovered its appeal: beautifully preserved Old World architecture, a varied landscape dotted with medieval towns, underrated cuisine and lovely, hospitable people. Flights are being added to meet the booming demand, and the hotels, restaurants and shops are becoming much more sophisticated. The long lag in development means the Portuguese remain refreshingly unpretentious, despite their proud emphasis on their country’s products — from wine and olive oil to wool and ceramics — and cultural heritage.