The Frankfurt market is supposed to be good, and while Hamburg is the best city in the world (no bias here!), our markets are underwhelming. For what to see/do in Hamburg (besides the Elbphilharmonie) we would need to know more about your interests. Are you a Rote Flora person? A Große Freiheit person? The Nikolai, the Alster, the Reeperbahn, Beatles Platz, Sternschanze, but also Speicherstadt, Landungsbrücken, St. Pauli / Fischmarkt, Blankenese, Airbus could be interesting. The nearby towns of Glückstadt, State, Lüneburg, Wilster or the nature of St. Peter-Ording or the Baltic. . . . all of these are great if you are based in Hamburg. Bergen-Belsen is also not far if you wish to see a KZ. There is a TON to see and do up here even in December, and knowing what your interests are will help.
For markets, I would recommend Bremen and Lübeck up in this area, with Bremen being our personal favourite. The train ride for either is average in terms of "scenic," but short and easy. We go to Bremen at least once every year for the markets, sometimes more than once.
Germany in December is only going to be "scenic" in deepest Bavaria or maybe along the Rhein. There is beauty in the moors of the north in the winter, but a bleak, desolate beauty that isn't for everyone and is generally not considered "scenic." I love it, but it isn't generally what tourists seek. Also, keep in mind that days are very short here in December (dark by 4:30), so "scenic" train rides would have to be enjoyed in the middle of the day and thus eat up one of your travel days.
