Three weeks from Berlin to Dusseldorf?

I'd start that trip staying 4 days in Berlin to really enjoy the city without the usual haste people visit it.

What, no Italy? Benjo with cheap, very fast, trains and frequent, cheap discount airlines covering all of Europe, you could go anywhere (pretty much) in Europe in 3 weeks. You acknowledge the wideness of your question. But you haven't given us here anything to hang our hat on. It is difficult to suggest where would be best for you without lots of more information from you. Travel is not a one-size-fits-all exercise. Anybody can give you a list of things they think you "must do". It will be based on their preferences and likes, not yours nor your wife's. Please tell us what sort of things interest the two of you. What sort of holiday do you want to have? Go, go, go and check off things as you see them through a window, or quiet - basing yourself in one or two places with perhaps a few day-trips to nearby places? Museums in detail or in a beach-chair on the sand with a drink? Walking in the mountains or working out in a gym? Sightseeing or eating and drinking? Early to bed or clubbing? Spas and saunas or riding trains? You see what I mean? What did you like or dislike in Spain (where?) and Portugal (where?)? Other than that you can use miles getting there, why Berlin or Dusseldorf? If one or the other, which one? Why? What about Prague, Bavaria (where? It is a big place?), Budapest, Amsterdam?
What intrigues you about them? And do you and your wife speak as one voice, or do we need to help you meet different needs for both of you? Congratulations on getting through your Masters (soon).

I'd probably choose from a couple options, both A and B with a rented or leased car (basic cars are cheaper to rent, bigger/nicer cars are cheaper to lease), B2 by train and ferry/cruise ship: Option A: Start south out of Berlin to pick up, at least, Munich and Salzburg, maybe some Switzerland (if your budget can stand it), a loop through part of France. If you haven't killed your time by then, you could come back up through Belgium and the Netherlands, then back to Koln and finally Dusseldorf for your ride home. Add or subtract destinations to your taste. Option B, B2: Make it a northern swing into Scandinavia by car or, B2, get on a cruise or ferry hops from Warnemunde to cover the Baltic Capitals by sea. After the cruise, you can come back down through Hamburg then Koln and finish in Dusseldorf. I tend to try to create logical swaths through an area and minimize the backtracking - which is why I'd choose north or south, not both.

Another option by train. Start south through Prague, Vienna, Budapest, and continue down to Croatia. From there pick up a cheap Euro flight back to either Berlin or Dusseldorf for your flight home (unless you can find a flight, using points, from that area). If you have time, both Koln and Hamburg are convenient by train from Dusseldorf.

Hi, With three full weeks flying into Berlin and dep from Düsseldorf, I see you have two basic choices, both of which are doable. First, you make this an all German trip, ie., just stay in Germany for the entire three weeks, or, secondly, include some outside cities such as Budapest, Prague, or Paris in your intinerary. What about Vienna? If you choose an all German trip, then I suggest seeing Berlin, Potsdam, Leipzig or Dresden, Hamburg, or Frankfurt, and Munich...yes, alot of choices even if you just pick big cities. For a place to get away from it all, say, between Berlin and Düsseldorf, I suggest Minden an der Weser.