Liminalbob

Frankfurt is a major international travel hub, so your flight will probably go here no matter what (assuming you're coming from North America).

Frankfurt is kind of lacklustre though, however if your main point of going there is to visit friends, then that's good and they can surely bring you to some interesting places with good food (and beer!)

Cologne is a more aesthetic city and if you have friends there too, then the sentiments above apply again.

Berlin is fascinating, and Germany is a small country, and the train system works perfectly and quickly; so, I don't know your time or budget, but it would seem remiss to not visit Berlin, perhaps for some downtime with your partner at the end of your trip.

One note though, if you are American, avoid talking about that or politics at any cost. Americans are seriously disrespected everywhere now, especially, truly especially, if you talk about the military. If you want to experience German (or any) culture outside of an expat community, do not mention you are an "ami" or that you are there to visit a military friend. You will almost certainly be "socially distanced".

(This thread is a few years old and my response is not directly tied to OP but the only way I could see to post)

I think most people consider cruises "trashy" is because you're choosing to be on big prison with 100->1000+ other people who are choosing to do the same thing, and, there's really nothing to see except ocean. OK sure, you have "ports of call" or whatever, but if you're "in Mexico" for 6-8 hours, that's no time to make friends, try more than one touristy restaurant, learn any language, it's not much better than a layover in the airport. People who say "I was on a cruise and went to Mexico (or wherever)", the audience is probably snickering at your naiivety because no, that doesn't count.

The only "cruise" I've done was in Ha-Long Bay in Vietnam, which is a fascinatingly beautiful area, UNESCO world heritage site, and can only be seen from the water, ie. a cruise. But the boats can only service about 30-40 people, including staff, so it's not what people normally think of, when they hear "cruise". Yes it was quite luxurious, my room was as good or better than most hotel rooms, there were only about 10 guests on the boat at any time, and it isn't a crowded party ship although there is a bar and you can do whatever you want; and there are scheduled activities. I highly recommend this option to any traveller - you will not find walmart level cruise ship "trash" on these boats as just to get to Vietnam is more money than most americans will ever have.