1. What is your budget?

2. How many days do you roughly plan to be there?

3. Would you like to city hop or be more focused on one region in which you can reach stuff by train?

If you are interested in city and urban planing I would suggest to def. make a stop in the Netherlands and have a real view of the biking infrastructure, sure big city is nice, but so is to view one other cities.

Besides that if you are in Germany there are some thing you could do and look at depending on your likings. If you likings are squats Barcelona is also a good place to be btw. You have as rough typology Western vs Eastern cities, you have Smaller vs Big ones and those that are relatively new built in terms of center (cause Bomber Harris bombed the inner cities to make space for new post WW2 urban development) vs those which city centers are old (e.g. Aachen).

So West vs East, Small vs Big, Old vs New. You could also add more, like Bochum in Western Germany is part of the Blue Banana (NSFW picture and in the Ruhr area where a lot of industry was and the population centers are quite interconnected. However it wouldn't add so much, since as tourist you will see new stuff anyhow, cause you don't know stuff.

For Germany I would urge you to look at some (former) squats, to have a look at some housing coops (ie cluster housing), at older housing of the GDR and maybe drive around a bit via train in the local city you choose. In Berlin it is traditional to go to the Tempelhofer Feld - large empty space in the middle of Berlin, a former airport and former KZ - and experience a lot of wind and SPACE!!! in the middle of a city.

It is much more easy to walk by foot in Leipzig and Dreseden than it is in Berlin, but if you would just focus on some areas in Berlin you can easily get around as pedestrian plus public transport.

Honestly I couldn't really tell where to go and what to do without having some more insight into what you wanna do. Munich and surroundings has some intriguing parts but its quite expensive compared to Leipzig.

Of course if you have 1-2 days and time to process and feel like it it could also be something to visit some historic places and learn about the people who survived, who were exterminated and how they resisted the Shoa by taking some tour at some now memorial places. They typically are a bit wider and also incorporate violence against others, e.g. Roma and Sinti.

Currently cause the Covid counts are rising I guess that in December there will be a lot closed.