Sorry for the late reply... I sort of forgot about TER for a while.

So... one added thing that is going to be either a hurdle or not, is your method of transportation/the amount of time you have 'on shore'.

Let's start in the north (all clubs I mention can be easily found on Google Maps) with Duesseldorf... I don't think any cruise will stop in Duisburg, some good clubs there but mostly towards the east.

In Duesseldorf there are four clubs that are (almost) within city limits, or at least easily reachable without a car. Three of these clubs are real 'evening' clubs with many young girls, louder music and you are less certain to get goodservice. Dolce Vita and Magnum are clubs I personally avoid, I just have never left satisfied there. Livingroom (to the west of Duesseldorf, but reachable though a 15 min train ride and a short walk, they also have a hotel closeby) falls in the same category, but they seem to have a few more 'experienced' girls. A very well run club with good food and some very nice girls, even though it is kind of small (but full of girls). Expect at least 40 girls (60 in weekends).

The best place to go to is Haus Panthera to the north of Duesseldorf. This club has a few older WG's, but even the young ones know what they are doing. Food is a la carte (included in price), there is beer, reasonable wellness... but it is the girls you come for, maybe not as modelly as in other clubs, but cute girls that all DFK a lot, CIM will be welcomed and a lot more happens. If you go in the weekends you'll notice quite a bit of public action (especially in the summer outside). Usually about 25 girls though, but I usually leave with a longer to do list than I started with. One of my favorite clubs.

Then there are a few clubs farther away towards the Dutch border, but they are difficult to get to and somewaht similar to Livingroom (Golden Time, Villa Vertigo and PLanet Happy Garden).

In the Cologne area, you'll probably need a cab or car. Parksauna Rezidenz in Lohmar seems to be exactly what you are looking for. Good wellness (although the whirlpool is a bit weak), good food (a la carte), a very relaxed and laid back club. The amount of girls will depend on when you go, but I doubt it ever falls below 30. Most girls in mid/late twenties, which also make them (on average) a bit less stunning than some of the 21 year olds elsewhere. It has a reputation of high service levels.

If you have to stay within the city there are two clubs that are very reachable by train, both from the same owner. In your case I would skip Mondial, there are a lot of very inexperienced girls here. Service is not a given and especially DFK (maybe also CIM) can be difficult to find. If you do find a girl that 'likes you', it can be great of course. Sister club Samya is a better bet (go to the website for pictures... yes they are all real, with only some tattoos being edited). In essence you still have to be a bit careful about service - because there still are a lot of young girls - but there also a few older girls that provide very good service (and some of the young ones as well off course!!). This club has two faces though, at day it is very boring and at night it fills up and it is one big bustling party with girls dancing on the bar. I have the best times here (even if the sex itself isn't always great). For both these clubs you should only consider them if you are able to stay late; otherwise you miss everything that makes the clubs worth going to. Both clubs will have more than 50 girls for sure.

Between Cologne and Frankfurt (if you get off there) there is really only one club: Finca Erotica in Dierdorf. Th clubs is nice with enough girls (also some more experienced ones) and finally you get to the part of the country where the girls are actually nude all the time (in the other clubs it is lingerie or topless). Not easy to get to, but a nice club.

Frankfurt (if the cruise goes there), here you really only find huge clubs with many girls. Oase and World are huge clubs (100+ girls) but they are kind of far away from the rhine. If you have the chance to go to Darmstadt, Sharks is a great club to go to (also 100 girls I'd think). In all the clubs here you have to a bit careful about service, but I think most girls will be happy to do CIM. These three clubs form the best 'o my god, I'm in heaven' feeling for first timers; Sharks is currently the club with the best 'line up' if locals are to be believed.  

If you have to stay in the city, there is Mainhattan and Palace. Mainhattan is nice enough and a bit smaller (still>30 though, easily). Palace has more experienced/older girls, but they tend to try to upsell. I dislike it intensely, but I know that guys that easily extend sessions or do CIM get great service here. If you just want a basic 30 minutes, you might have a lesser experience.

I'm not as familiar to the clubs to the south, but there are a few there