Day trip from Vienna to Melk/Kelms to ride bicycles on Danube

The best. i.e. direct, train route from Vienna to Melk is departing from Wien Westbahnhof (western train station).
Similarly, from Krems to Vienna the direct trains arrive at Wien Franz-Josefs-Bahnhof, but you might alight one stop earlier at Wien Spitttelau to catch the subway U4 or U6.

Both routes are served by CJX trains (CityJet Express). For these trains tickets cannot be bought more than 60 days in advance, but there is no need to do so anyway, as seat reservations are not available in this kind of semi-local trains. You may buy your tickets just prior departure from a vending machine.

The Einfach-Raus ticket is a good option, but your proposed travel days are work days, i.e. you may not use this ticket before 9am.

A little complication regarding the bike route on the south (right) embankment of the Danube.
https://community.ricksteves.com/travel-forum/austria/melk-to-durnstein-by-bike-hike-durnstein-to-krems#2336872

Bummer on the landslide as I was thinking about switching over to other side at Spitz. I did read you can switch to other side at Melk but it was more on the street. Does anyone have experience riding on the other side (opposite side of Melk)? If so how busy is it on that side?

I am wondering if this is even worth doing at this time. I also saw the possibility of doing a cruise. Any experience on the cruises? Thanks

The landslide shouldn't keep you from your planned route. As you can see from the provided map there is a temporary ferry around the landslide for pedestrians and bikers and a shuttle bus for bikers only.

In Melk there is a road bridge for crossing the river Danube.

Here you can find all details for biking along the river Danube:
https://www.donau.com/en/wachau-nibelungengau-kremstal/outings-activities/exercise/danube-bike-path/

Ok, I do see this on the map. I am assuming bikes can go on both the ferry and shuttle? I wonder by October that the landslide might be cleared, fingers crossed :) Thanks.

I wonder by October that the landslide might be cleared, ...

No way, it is expected that the road will be opened again in 2025. Before the road can be cleared loose rocks above on the slope have to be removed, and the ground must be fixed somehow, otherwise the next hard rain will cause more mud and stones coming down.

I have ridden on both sides of the Danube down to the Spitz ferry. They are both nice rides with not much traffic where you will ride. When the road is busy, there is typically a separated bike path.

I have twice taken the boat back up from Krems to Melk and it's a pleasant ride, but as I recall there are fewer boats later in the season, so you will want to look at the website in advance for times.

Though the ferry may be irrelevant at this point, you can pay right on the ferry, and the crossing doesn't take long. You may have to wait for it to get to your side and be ready to leave again.

If you are staying in Melk, your hotel may have bikes, or you may get a rental company to deliver some for you to your hotel which you can then lock up at the boat dock or train station in Krems. I have done both and the rental bikes were better quality than the hotel bikes. I have not used Nextbike.