Our itinerary has us flying into TXL and out of CPH in late March. We've spent lots of time in Berlin but will probably spend at least a day or 2 there to begin. As for CPH, while we would never rule anything out our initial desire is to spend around 3 days there, and no more- largely due to budgetary and climate reasons. Between those two cities we have around 5 days. We are open to trains or renting a car, however flights are not an option. Was hoping to just get some general ideas; these planning times are almost as fun as the trip itself. Some thoughts:

We have been to Dresden and Munich.

Hamburg is certainly quite well-situated logistically, either for a train trip to CPH and/or possibly dropping a car there before doing so.

Prague (and/or any smaller Czech towns) is an attractive option- I have spent considerable time there however my wife has not and would like to. Primary concern here is it seems a bit out of the way if we are relying on trains to get down to Prague and then back up to CPH. A car would seem mush more suitable here.

Places like Vienna, Bratislava, Warsaw, etc all seem a bit too far.

We've heard good things about Poznan, and it seems fairly simple to get to and from.

Any other suggestions for the eastern part of Germany (either big cities or small towns), western area of Poland, or the Czech Republic? Ideal driving routes, trains, etc.? Or should we brave the colder temps and spend the extra money and devore more time to Denmark and perhaps some of Sweden? Thanks in advance.

If flights are not an option, you would lose a lot of time in going back from the Czech Republic or Poland (unless there is ferry that goes from Poland to Copenhagen)to Copenhagen. Between Hamburg and Lübeck you should be able to fill in your days. The Michelin Green Guide for Germany might be of some help.

Potsdam is worth at least one day (Sanssouci, New Palace, Alexandrowka, Dutch quarter).
http://en.wikipedia.org/wiki/Sanssouci

I don't understand some of the logic here: Prague isn't any more "out of the way" if you get there with a CAR than it is if you get there using the train and with both forms of transport you are going to do some backtracking geographically.

If Prague is NOt an option I would agree that Hamburg and Lubeck could be of interest if not Copenhagen itself.

You could explore the Baltic Coast (Stralsund, Wismar, Schwerin and Lübeck). Kiel isn't to my liking (having been badly damaged through the war, the architecture is too modern for me).

Lavandula

Definitely hit the Baltic coast. Rügen Island (Binz, Sellin, the National Park with the chalk cliffs, Putbus, the steam train), Stralsund, Wismar, Lübeck as UNESCO world heritage sites (brickstone architecture).

Schwerin is a beautiful small city by the lake with very nice palace.

And then there's Hamburg.

With only 5 days I wouldn't bother going in the opposite direction (Prague) or too far out of the way (Poznan).

I.

On the Deutsche Bahn website you should be able to get from Hamburg to Copenhagen in under 5 hours if you take an ICE train. They are much faster than driving. Hamburg and Berlin are less than 2 hours apart by ICE.

Hamburg is so lovely, it's nice just to walk around. I would spend 2 nights there so you have a full day to go on a tour (Sandemans free tour is great) and see one or two more places like a church and a museum.

Side note - I am biased because I live near Hamburg. If you want palaces or castles you should go elsewhere but Hamburg has enough churches and museums to fill a few delightful days.

thanks for the replies so far. The northern coast sounds great, and should work with either a car or the wonderful DB. My only concern (and I know this is difficult to ever "know") is if some of those northern town would be quite cold and this harder to appreciate the nature/outdoorsy stuff?

But thank you for the motivation- without it i might be far too tempted to just spend the whole week in Berlin.

Cold is relative Be prepared for (cold) wind right on the coast (chalk cliffs, Binz and Selling e.g.) Not so much in the bigger towns like Stralsund, Wismar, Lübeck. Plan your activities according to the weather forecast - if the wind blows from the sea then I'd reduce time right there. Nobody knows how the weather will be ...

It's unlikely to be colder than Berlin. Berlin has continental climate, which implies cold temperatures in winter, warm in summer and low humidity, which makes both rather bearable.
The coast (including Hamburg) is likely to be warmer, but due to higher humidity may actually feel more chilly. Late March could be either spring or still a bit winter-like.

While I might be tempted to choose one or two nights in Hamburg and combine it with Luebeck, if you head to the coast you have several options:
I think Ruegen is too big to make sense in such a short-time combination with Hamburg.
So you could do Ruegen or the Darss peninsula in combination with Rostock (and possibly a quick tour of the impressive red brick cathedral and monastery in Bad Doberan) and then cross by ferry from Rostock to Gedser on Falster and then proceed to Copenhagen. This route may be more complicated with regards to a rental car, though.
Or you could hit the coast at Rostock and head West to Luebeck and then proceed via ferry from Puttgarden to Roedby.

Should you decide on Hamburg (+Luebeck), you could keep your options with regards to either using a train the whole way to Copenhagen or driving.