The Classic Downstream Loop: Vienna → Melk → Dürnstein → Krems → Vienna
Because the Danube flows fast from west to east through the Wachau gorge, always start at the western end (Melk) and work your way downstream to Dürnstein and Krems an der Donau. Buy the all-in-one ÖBB Wachau Kombi-Ticket (€69) at any ÖBB ticket machine in Vienna: it includes your morning train to Melk, entry to Benedictine Stift Melk, the 1h 40m downstream DDSG Blue Danube boat ride past Spitz to Dürnstein, and your evening REX4 train from Krems back to Vienna Franz-Josefs-Bahnhof.