We'd booked a bit of a tour for today. You can buy a "combo" ticket for a train from Vienna to Melk, entrance to the Abbey, a river cruise down the Danube, then a train from Krems back to Vienna. It's pretty touristy, but it's a nice way to see some of the Wachau Valley if you don't have much time (which we didn't).
The Abbey was grandiose and pretty, including the gardens and the view, and the river cruise was pleasant -- if windy and a bit cold on the exposed top of the boat.
I accidentally took a few photos in the library of the abbey before I was told that wasn't allowed. It was such a gorgeous space, and I'm (not-so-secretly) glad I got a few shots off, haha. (Incidentally, the church was just as gorgeous -- a bit ostentatious even, maybe -- and photos sans flash are allowed in there.)
I also had a fantastic lunch in Melk before we boarded the boat at the restaurant of a hotel on the main drag: pumpkin gnocchi with zucchini, peppers, arugula, and balsamic. More pumpkin -- yum!
We had about 45 minutes to wander through Krems before boarding our train back to Vienna, and their old town is quaint, but nothing stood out. We had hot tea and goulash soup to warm up at a place called Cafe Berger after the breezy boat ride.
A fun little day trip from Vienna if you're looking to get out of the city.