Family of 3 spent 2 days in early May in Krems and Wachau Valley. I didn't see a lot of information on this topic before we left. This was our experience. We rented bikes from Wachau Explorer for 24 hours. It was close to the Krems train station and they were easy to work with. I was worried about 1) Safety riding on the busier north side of the river. 2) Biking up stream/up hill. They assured me traveling from Krems to Melk on the north side of the river was the best and most scenic route. 1) Ride was safe. I'm 60 yo and not a regular bike rider. Ride was not difficult and always felt safe. We were never traveling on the main road. Majority of ride was on a bike path, wine field access road, through cobble stone town centers. The times we were next to the road there was a curb between us and the road with a wide area to ride safely. Saw many young kids and family riding this route. Helmets were 2 euros to add, which we did. chain locks were provided. 2) It never felt like were were going up hill. We rented regular bikes (not eBikes). There was some up and downs through towns but all easy.
I will also add we had great weather. It was one of the best days of my life. The scenery, historic small beautiful town, terraced fields of grapes and places to eat and drink. We had made many stops and decided to take the hour long small local train that travels from Emmersdorf (across from Melk) to Krems with many stops along the way. It was 20-24 euros each, no cost for the bikes. There are bike racks on the train. It was a nice trip but if have time to ride back I would recommend that and add a boat crossing for the experience.
The Nextbike option mentioned in some posts, my US phone number didn't work to set up the App for it Others I met also mentioned this.
We bused up the north side again the next day and did several hikes in the Durnstein area and bused to visit Melk. FYI, the full day bus pass on the north side of the river didn't work for the south river side bus route to Krems (or at least that is what the bus driver told us).
It was early season and mid week. We didn't see any wineries open, we stopped for food in the towns we visited.