The hook
Forty kilometres of the Danube between two small towns, terraced with vineyards that have been worked since the Middle Ages, with a vast yellow baroque abbey at one end and a ruined castle where Richard the Lionheart was imprisoned in the middle — and a flat cycle path the whole way.
01Why it's worth it
The Wachau is the easiest good day trip in Austria, and it is easy in a way that feels slightly unfair: an hour from Vienna, one ticket, no planning.
Melk Abbey anchors the upstream end — an enormous Benedictine monastery on a rock above the river, baroque to the point of excess, with a library and a marble hall worth the entry on their own. From there the river runs down past terraced vineyards held up by dry stone walls, apricot orchards, and a string of small towns: Spitz, Weissenkirchen, and Dürnstein with its blue-and-white church tower and the castle ruin above it where Richard I was held for ransom on his way back from crusade in 1192.
The wine is the local seriousness. Grüner Veltliner and Riesling from these terraces are among Austria's best, and the Heurigen — growers' taverns, open when the grower says so — are where you drink them, often in a courtyard with cold food and no menu.
The genuinely good move is to combine the transport rather than pick one. Train to Melk, abbey in the morning, boat down the river in the afternoon, train home from Krems. Or hire a bike and ride the riverside path, which is flat, separated from traffic, and one of the pleasanter days on a bicycle in Europe.
02How to experience it
Buy the combined ticket in Vienna, which bundles train, abbey entry and the river cruise. Go upstream to Melk first so the boat runs with the current. Get off in Dürnstein if the boat allows, climb to the castle ruin for the valley view, and eat at a Heuriger before the train back from Krems.
From the locals
The Austrians in the r/travel country thread mostly want to correct an assumption. One who spent a year living in Burgenland put it plainly: "don't assume Austria's just skiing and Vienna — the Danube Valley and Wachau are beautiful, as is the rolling hilly country of southern Burgenland and Styria."
The practical version came from a visitor who had done exactly the loop described above: "one of the more fun things we did was a day trip on the Kombiticket. With that we got a train ticket to the village of Melk, admission to the beautiful and huge Melk Abbey, a ticket for an afternoon cruise down the Danube's Wachau Valley that had a narrated tour of the sites in English, and train tickets back to Vienna from the village of Krems." They add that the same ticket can cover bike hire between villages.
One standing caution for the trains involved, from the same country's threads: "be very careful with the public transport system — it is very efficient but works on a trust system, so make sure you VERIFY your tickets correctly, because they have a lot of ticket inspections and you will be fined."