Luxembourg, a small yet influential country in the heart of Europe, offers visitors a rich blend of historic sites and contemporary culture. The capital city, Luxembourg City, is famous for its dramatic cliffs and ancient fortifications that have earned it a UNESCO World Heritage status. Walking through the old quarters, visitors can explore the casemates that reveal layers of the city’s military past.

ABERDEEN -- Scotland's third-largest city (pop. 187,000) is characterized by buildings of beautiful white granite. Set beside the North Sea, Aberdeen is both a university town and the capital of the Scottish oil boom. It is also one of Scotland's largest seaside resorts (though its appeal is confined to those who like their water well-chilled).

Oslo, Norway’s capital, stands at the crossroads of Nordic history and forward-thinking design. Originally founded over a thousand years ago by Viking King Harald Hardrada, the city has evolved from a medieval trading hub into one of Europe’s most modern capitals. Visitors can explore its layered past at the Akershus Fortress, a 13th-century stronghold still standing guard over Oslofjord, or walk through the preserved wooden homes of Damstredet.

Opatija, located on Croatia’s Kvarner Bay, has been drawing visitors since the 19th century, when Austro-Hungarian aristocrats built grand villas along its Adriatic shoreline. Today, many of those same buildings still stand with some restored as luxury hotels and others preserved as cultural landmarks. The town’s most recognizable structure, Villa Angiolina, opened in 1844 and marked the start of Opatija’s rise as a fashionable seaside resort.

One of the major rivers of the Iberian Peninsula, the Douro in north-central Spain and Portugal just might be the world's most beautiful wine region. Most of the river on the Spanish side isn't navigable, so river cruises generally travel roundtrip from Porto, on the coast of the Atlantic.