I have a car and need to get from Copenhagen to Amsterdam. I would like to break the trip into two days. Any recommendations as to a stopover worth seeing in northern germany?
Hamburg and Bremen are both roughly mid-way, you'll come to Hamburg first. One of the finest baroque churches in northern Germany is the Hauptkirche St. Michaelis; also the Rathaus ("house of the political rats"); and the Kunsthalle, probably northern Germany's leading art museum. And, if you're an unaccompanied single gentleman, it's possible Hamburg would be of special interest for more than just its cultural offerings, something not written up by Rick, you can learn for whom the Hamburg beer St. Pauli Girl was named (google the Reeperbahn or PM me for more on that).
I like Hamburg. Take a boat ride that includes the old warehouse district. Walk through St. Pauli (remember when you thought the St. Pauli Girl beer label was a milkmaid? It was the red light district, now more of an upscale shopping area). See churches where Martin Luther preached.
My favorite is the Rathaus. They have a painting of every burgermeister going back to the Middle Ages. One portrait, probably painted in the 60's, has a very green looking burgermeister sitting next to (presumably) his wife who has a huge scowl and is smoking a cigarette in it's Cruella deVille style holder. (Our joke was the guy died before they could do his portrait so she is posing with her dead husband and not at all happy about it.)
I wouldn't rearrange a trip to see Hamburg but it's a great city with plenty to see.