German Travel: Looking to visit Frankfurt and Cologne and/or surrounding areas in future

Not been to Cologne but Frankfurt is probably the least interesting place in Europe. There's really not much there at all. If you're going to be in Germany I'd suggest sticking to Bavaria.

Heidelberg
Rhine boat castle tour (can do anything from a few hours to a whole day)
Wertheim Village (shopping)
Neuschwanstein Castle
Strasbourg (really enjoyed this city)

ETA: Nuremberg is also a cool city

I have been to Frankfurt. A few years ago we took a tour of the Alpine countries - southern Germany, Austria, Switzerland and the Dolomites and Lake Region of Italy. The reason we went to Frankfurt was because we flew in and out of the airport there. It was a great trip, although I would not consider Frankfurt one of the major highlights. The main thing we did in Frankfurt was hang around Romerburg Square. It is in a nice, Old Town area right near the river with shops, cafes and beer halls. Fun place to hang around for a few hours. There is a beautiful cathedral a couple of blocks away. I have never been to Cologne, but my suggestion is to spend a little time in Frankfurt and try to explore other areas in Germany.

If you focus on visiting friends and don’t think of this as an epic Germany trip, you’ll be fine. Frankfurt isn’t old world scenic, but you can still find lots of good German food and beer. Let them show you around their neighborhood, their fave spots, and then plan a weekend excursion to a smaller, more scenic town. If Frankfurt is your base, Strasbourg is only 1.5 hrs by train—you could sneak in a bit of France.

Even everyday life in another place can be deeply interesting, if you are curious and open minded.

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".

I have spent plenty of time in Europe and know when to not talk about politics.

I usually let them talk and make non-committal comments. In my experience, Europeans are extremely fascinated by American Politics and love to give their opinions and find out what we think.

I'm not really worried about that.

So my friend lives right outside of Kaiserslautern actually. I spoke to him and he has spent very very little time in Frankfurt, so I doubt we would spend much if we can and visited.