Europe's second-longest river flows through 10 countries, although cruises typically travel along the stretch between Budapest and Regensburg, with Passau, Germany as a common starting and ending point. On a classic weeklong trip, you'll sail along the borders of Hungary and Slovakia before passing through northern Austria and southern Germany — this is the "greatest hits" voyage, with stops at capital cities, vineyard-covered valleys, and standout cultural landmarks. Other itineraries extend as far the Black Sea, sailing through Serbia, Romania, and Bulgaria.
The cruise season runs March through December. There's a risk of high water in springtime and low water in the fall, though river levels vary year to year.