One the best day trips from Vienna is exploring the Wachau Valley of the Danube River, which stretches from Melk to Krems. The day trip can easily be made into a 2-3 day side trip.
There are substantial hills flanking the river on both sides which have been carefully terraced into vineyards. Fortress ruins sit on top of many of the higher peaks, including the famous ruin of Duernstein where Richard the Lionheart was held for ransom.
While Melk and the even larger Krems are nearly worthy of a day visit on their own (the Melk Abbey is extraordinary and the town of Krems is chocolate box perfect), I focused this visit on the area between these two towns - an area full of villages with wineries, gothic churches, apricot orchards and an excellent biking path.
This region is approximately 1.5 hours by train from Vienna. I did this day trip on April 4, as the skies were blue, the winds were mild and the apricot trees were in full bloom (if you want to see my photos from the day trip, please have a look at my Instagram account @3point14maker). I've done this trip more times than I can count and thought I would provide some wisdom here.
Getting There and Back - As mentioned above, this area is accessed by train. Use Google Maps public transportation function to determine the best route from your location in Vienna. For me, I took a train (OeBB Railjet) from Wien Hauptbahnhof to St Pölten where I switched to a local train to Melk. I bought my ticket at the station just before I boarded for 21.60 Euro (one way). For my return from Krems, I took a regional train from Krems to Wien Heiligenstadt where I then boarded the U4 subway line to get home. Total return cost was 21.60 Euro as well. As I was traveling as a single person I paid full fare each, but had I been with my family I would have used my Voteilscard Family and saved a bit or (more practical for non-residents) is the Einfach Raus Ticket which allows you to travel on regional trains as a group for a discounted price (NOTE - this ticket would not have worked on my Railjet train from Hauptbahnhof to St. Polten). Details on Einfach Raus tickets can be found here - https://www.oebb.at/en/tickets-kundenkarten/freizeit-urlaub/einfach-raus-ticket
Biking - I use the NextBike App, which allows me to use my phone to scan QR codes to rent bikes. This app works in both the Wachau and within Vienna. It is very cheap - I rent bikes regularly in Vienna for less than 1 Euro for nearly an hour. In the Wachau I had the bike for nearly 7 hours and it was 11.25 Euro. It is important to know that you need access to a smartphone and data in order to use these bikes. The NextBikes are lined up at a stand at the Melk train station and there is another stand at the train station in Krems. With the NextBike app, you can see that there are NextBike stands in every village in the Wachau and numerous stands in the larger towns. The bike has an adjustable seat, a basket and is in great condition. If helmets are important to you, then you might want to bring one. That said, they are heavier bikes not the sleek ones for long distance. I took my bike at 10:00 am and returned it at approximately 5:00 pm. Google Maps said that the bike journey I took was a little over 42 kilometers, which would have taken about 2.5 hours if I biked straight through. But I stopped many places for photos and for lunch, so a slower way.
The Route - I took my bike as mentioned in Melk and biked all the way to Krems. I took the north bank of the river the entire way. It is possible to also take the south bank or to cross the river at ferries at Spitz, Weissenkirchen or Duernstein. I prefer the north bank as that stretch includes the villages that I love, whereas the south bank is more agricultural. The bike path is part of a larger bike path that follows the Danube for hundreds of kilometers. The path is entirely protected from traffic with the exception of the stretches within Melk and Krems - so very safe.