Will be spending 6 nights in Berlin -- mostly likely December. Other than Hamburg, what other day trips would you recommend?

Personally, I wouldn't consider Hamburg as a day trip from Berlin. Possibly Dresden... although it's worth more than one day and Wittenberg for starters. Definitely Potsdam.

There's so much in Berlin that you wouldn't have to leave at all during that time.

day trips I have enjoyed

Potsdam and Sans Soucci Palace - the Prussian Versailles

Lutherstadt-Wittemburg

Lubbenau for boat rides thru canals - actually punts pushede by punters

Schwerin

last two really neat old historic cities - Schwerin is on a lake and has a castle on a lake.

If you want to see a taste of Poland do like I did hop the train and in about an hour was in a Polish town - rather drab but typically Poland.

You have got excellent advice - for summer!

In December, many of these options do not really work. The Potsdam gardens are bleak, the canals are frozen and even Schwerin Castle has lost much charm.

Six nights mean five days, and Berlin has lots to offer, so I would not recommend to leave the city for more than one day trip. My first choice would probably be Dresden - there is a direct train that takes 2:06. To Hamburg, the train ride is just 1:40. Lutherstadt Wittenberg is just 41 minutes from Berlin.

traveller, is correct. First question: Have you been to Berlin?

Why would you go all the way to Hamburg? There are plenty of day trips out of Berlin that aren't as far.

There's plenty to do in Berlin.

You're really talking 5 days and those are going to be short days at that time of year.