Danube River cruises between Amsterdam and Budapest are a terrific way to experience the span of Europe and in essence to explore two great rivers, for the western third of the itinerary is along the Rhine, which has been connected via the Main-Danube Canal since 1992.
Travelling west to east, these grand tours usually last more than two weeks and would leave the Rhine on day four or five, turning left on to the Main River at Mainz.
There are a host of smaller market towns amid the castles and vineyards of southern Germany, so not only is there plenty to see but it spreads the load of river-ship passengers coming through, particularly during the height of summer.
Many cruises stop at the Bavarian valley town of Miltenberg, about 60km south of Frankfurt, for a visit to its castles or breweries.
Alternatively, some cruises pull over at Offenbach, part of the Frankfurt metro area, where passengers can visit the city, tour the castle ruins at Heidelberg or see the 400-year-old Renaissance manor of Schloss Johannisburg at Aschaffenburg. There may be a chance to see the medieval town centre and taste the wines of Wertheim but definite on most itineraries is a stop at Wurzburg, for the Wurzburg Residence, one of the world's finest baroque palaces.
Bamberg has long been known for its smoked beer and, more recently, as the beginning of the Main-Danube Canal. This treat for lock-lovers stretches 171km to Kelheim and the Danube.
And it's anything but plain sailing. Because of the height difference between the two rivers, ships must traverse a series of 16 locks over about 160km, at one point reaching a height of more than 400m along the Continental Divide. Along the route, there's a visit to Nuremburg where passengers can see the site of the nazi rallies and Courtroom 600, where the post-World War II trials took place.
It is here that some of the week-long Danube-only cruises begin, while others start after the canal has merged with the river at Regensburg, one of Germany's best-preserved medieval cities, or at Passau on the border with Austria. Passau is known as the City of Three Rivers for the Danube is joined by the Inn and the Ilz. An excursion into Passau is a sound move, for the town is home to the 17,774-pipe organ of St Stephen's Cathedral.
But from Passau many cruises offer a great choice, not just of excursions but of countries, including a luxury train journey to Linz in Austria via Mozart's birthplace of Salzburg, and even a visit to the castle town of Cesky Krumlov just over the border in the Czech Republic.
A Danube cruise is a journey into the history of the Austro- Hungarian Empire - one of the world's great powers until only 100 years ago. Melk, dominated by a huge Benedictine abbey, is the usual first stop in Austria and there's often the choice of walking or cycling tours.
The abbey marks the beginning of the Wachau Valley between Melk and Krems, one of the few spectacles that can rival the Rhine Gorge. And, with the exception of the ruins of Burgruine Aggstein, the abbeys, castles and town gates of the Wachau Valley are better preserved than the forts of the Rhine.
Such is the glory of Vienna that one could spend weeks touring its landmarks, so it's best to choose a cruise that spends more than a day in the Austrian capital.
With no hope of fitting everything in, all itineraries offer choices including imperial history walking tours of the Ringstrasse and Schonbrunn Palace, visits to the Vienna State Opera and Museum of Fine Arts, time at the Spanish Riding School and even trips to the Slovak capital of Bratislava, just an hour to the east.
The cruise continues along the Slovak border before the Danube dips south to the Hungarian capital. Most cruises arrive before breakfast, allowing passengers a full day to visit the Grand Market and Karolyi Garden, explore the Hospital in the Rock, enjoy a recital at the Hungarian State Opera House or soak in the city's thermal baths.
Just as with the Rhine, the festive period transforms a cruise along the Danube. A week-long itinerary would include visits to the Christmas markets in Passau, Linz, Bratislava and Budapest, each with its own variations on spiced wine, gingerbread, pastries, folk art, wreaths, carol singing and nativity scenes.
With backdrops such as Melk Abbey and St Stephen's Cathedral, a spectacular light show in Linz and the likelihood of snow, Christmas on the Danube has an uber-traditional feel.