Rather than the cultural capitals that line better-known European waterways, a cruise along Portugal's Douro River offers lush hillsides and wild scenery. The cliff-like terraced vineyards are uniquely breathtaking. Voyages begin in Porto, traveling to Vega de Terron — just across the Spanish border — before working their way back to Porto. Trips last six to 14 days; the longer ones add land-based visits to Lisbon or Madrid. Sailing season is March through December, with the September harvest being prime time. Be prepared for foggy nights near Porto and scorching inland heat in summertime.