The further north, the clearer the pronounciation of German.
Bavaria or Baden-Württemberg (where Lake Constance is) has a very heavy accent, my DD who is bi-lingual defintiely struggled this Summer hearing some people.
Hamburg is a great city (I may be biased, DH is from there). You have lots ot see and do for a couple of days with a day trip to Lübeck. You can then either go on to the Baltic Sea or the Holsteinische Schweiz (a lake district north of Hamburg).
If you want more hills, the Fränkische Schweiz, Frankonian Switzerland, is good. That's the area north of Nürnberg, walks, history, good food, watersport on the rivers.
I would recommend getting a car though.
Another alternative is a couple of days in Berlin and then off to the Spreewald or - what we did - going down to Bavaria for some walking. We were at the Chiemsee, a long trip down but flew into Berlin, drove down and flew back from Munich. You can take the train though.