Rhine River Cruise

K-D is the biggest and best known. They have 4 or 5 runs a day. Depending on where you are staying, take a train to Ruedesheim or Bingen and board the boat there. It might be easiest to go straight through to St Goarshausen or Boppard and then return on the train, stopping off for shopping along the way. BTW, the K-D boats are giant restaurants, so stopping to eat is not necessary. A one-way cruise to Boppard is only about 3 hours and 45 minutes going that direction (down stream).
https://www.kdrhine.com/rhineschedule.htm

I had a good experience with KD line, which is the largest, most well established cruise line. Actually I'm not even sure if there is another one.

https://www.k-d.com/de/

You can start to cruise from Mainz, but the most scenic part starts from Rüdesheim and Bingen.

I give you the link to the timetables, because they managed to hide it quite well on their website! It took me a while to find it.

https://www.k-d.com/fileadmin/schiffstouren/KD_Fahrplan_intern_2019_web.pdf

Why do you must sail back? I mean it's a nice journey to repeat, but perhaps you could just take a train back. It's much faster and it also has some nice views of the river.

What town should we start out of?

Rüdesheim, since you're coming from Frankfurt and there are direct local trains to R'heim.

Bingen-Rüdesheimer Cruises offers several kinds of cruises, including the "Loreleyfahrt" ships at 11:00 and 13:00 - or take the KD cruise at :15 after 9, 10, 11, 14, or 16:00. (B-R does not cruise to Boppard, only as far as St. Goar. But that really is the most scenic part. Scroll to "Loreleyfahrt" schedule below for B-R cruises.)

https://www.bingen-ruedesheimer.de/pub/media/wysiwyg/Fahrplan_2019/Fahrplan_de/Fahrplan_2019.pdf

I would like to stop a time or two along the cruise to shop and eat, as well.

If you just stay on the boat to St. Goar (about 1 hr. 40 min. or so) it will be much easier - the skimpy cruise boat schedule makes it more difficult to stop off and to move on when you want to. Once you arrive in St. Goar - the home of Rheinfels Castle, btw - you'll find shops and restaurants there. And when you return to Frankfurt by train from St. Goar, the trains will stop in Oberwesel, Bacharach, and other towns - and these trains are VERY frequent by comparison with the cruise ships. You'll find it much easier to hop off and on in this manner. This route home to Frankfurt will take you via Mainz.

n_smedley, you are perhaps describing a tiny-boat private river excursion that is virtually never discussed here. I'm sure it exists for high-end travelers, but I don't know a particular one.

Because boating is much slower against the river current, cruising north (in the Middle Rhine, that is) is more efficient (and not fast enough to spoil the views.) The train home is far faster than the boat, if less scenic. But you don't need to see the same sights twice at boat-speed. You could get off the train, especially if you don't have luggage. You will want to download the full (seasonal-post your dates if you post again) schedule for the boat line you select. This will make it clearer to you that the boats do not have city-bus frequency, and should not be used for multiple stops.

The K-D line (which we took from Bingen to Boppard, not the route discussed above) has hot food and drink aboard, but of course it's not a gourmet restaurant. We had beer and sausages. I suppose our boat had a capacity of 200 or so, but it wasn't remotely full the day we went. If it were full, some people would be looking through windows from the indoor cabin-there isn't that amount of open-rail space.

Does your request for "return" routing mean that you are driving from your suburban stay? It would still be far faster to take the train back to the town where you parked and got on. Depending on your plans for the rest of the day, you could park in the ending town, but it all depends on where you want to have dinner and what you want to do. I would imagine that trains become less frequent after rush hour is over. If you don't have a car, you might have to take the S-Bahn into Frankfurt to get another train to your boat city.

Did you see this recent thread? It might be helpful:
https://community.ricksteves.com/travel-forum/germany/train-from-frankfort-to-bacharach-to-trier

I am also going to go to the Rhine tomorrow (26th Aug). I am going to travel by train to Rudersheim. I want to do the middle region and the Loreley. I don’t want to do a return trip. Once I have seen these sights which is the best stop to get off and either catch a train back to Frankfurt from there or do I have to return to Rudersheim? Any advice would be great.

The post just above from joy is a duplicate of one in her own name. This copy just woke up this old (April) thread. n_smedley is probably back from their adventure. I wonder how it went?

If it's the KD boat, you will pass the Loreley just before the boat reaches St. Goar; it then proceeds to St. Goarshausen, where you should get off for the train back to Frankfurt. The train stops in Rüdesheim on the way.

St. Goarshausen > Frankfurt train route: https://upload.wikimedia.org/wikipedia/de/9/91/VIAS-Netz.png

Check train schedules here: https://reiseauskunft.bahn.de//bin/query.exe/en

Thanks, all! I actually go in 2 1/2 weeks! The friends we are visiting decided to go with us and offered to drive, so we couldn't pass that up.
I went on a river cruise back in 1992 and loved it, but am going again so my husband can experience it.