The bulk of summer season has ended, and it's around six months prior to the new Rhine River cruises. Enough time to think and plan over an excursion. So my advice.
1. There is one major company (K-D), and roughly twenty minor companies running cruise ships along the Rhine. There are a couple for the Moselle as well. All have web sites, travel times, and cost listed. Plan around their schedule.
2. It's absolutely hectic to go on a cruise in July and August. So I advise you avoid this period, unless it's a Monday through Friday trip.
3. Length of a trip? You could actually show up at site #4, ride sixty minutes up to site #7, and get off there, returning to your destination by rail. You will find almost every Rhine port that a ship pulls into....has a railway station, and it's a simple deal to travel.
4. Get on early in the day. An example, K-D starts in Mainz in early morning hours, with the second stop in Bebrich (Wiesbaden). Neither has great parking advantages, and you'd be better off to travel there by rail or by charter bus.
5. Don't buy food on the cruise vessel. Generally, I can't think of any occasion that I got a three-or-four star lunch onboard. The beer, wine, and coffee are great.....but I'd take a pass on food.
6. Seating? In July and August on weekends....forget about it....it's standing room only. That's the reason why May and June are so appealing.
7. Weather? It can go from one extreme to another.....extremely hot and windy....or extremely cool and windy. Ask about the weather and be flexible on what day you make the trip. If you know it's going to be hot sweaty weather.....then I'd still bring along some wind-breaker jacket.
8. Rates? It goes from four Euro to seventy Euro (depending on where you get on and off, and if you want a return ticket).
9. Hint, the boat takes forty-percent longer going down the river, than going up. So, an entire day trip from Mainz to Koblenz, and back.....is an awful long trip. Kids won't be enthusiastic about such a trip.
10. Drinking. Well, folks get relaxed on such a trip and start drinking excessively. Guys do it....gals do it. And by the end of the trip.....you barely walk off the vessel and you might be drunk. In fact, too drunk to drive. So, utilizing a bus charter deal....would be strongly recommended. You might want to control your drinking a little....to avoid some really bad moments later.