A friend and I traveled to Northern Germany last September, primarily for a friend's wedding, but we spent the week prior to the wedding touring around small towns. Here is a report on what we liked and didn't like.
(1) Lubeck: We flew in to Hamburg and took the train straight to Lubeck. We took the local train to the main train station in Hamburg, and it was a little difficult because some work was being done on that line, and we wound up having to get off the local train and take a bus to another stop. My German is pretty rusty, and we had a hard time understanding the announcements on the train. It wasn't that big a deal, but when we came back to Hamburg at the end of the trip, we checked this online to make sure everything was fine.
The train to Lubeck was great--quick, clean, on time, etc. We stayed in the Park Hotel am Lindenplatz in Lubeck. This is between the train/bus station and the historic center, so it is very convenient. It was fine but not great. The rooms are quite large for Europe, and the bathroom was nice. The beds were just OK, and we had some trouble with the internet at first.
Lubeck is very pretty, but there isn't that much to see or do. One full day is plenty. We saw the Holstentor (did not go in the museum), went in the Marienkirche, went up the tower in St. Peter's (very nice view), went in the Heiligen-Geist Hospital (interesting but very small), and to the Europaisches Hansemuseum, which is new and very nicely done. We enjoyed the Cafe Niederegger and Cafe Maret for breakfast/cakes/snack time, and Schlumacher's and Brauberger for dinner. We also ate lunch at the old seafarers' union (I can't remember the name), and I would highly recommend that--it's a neat interior, and the food was good. The one thing I wanted to do that we didn't get to is Travemunde. It was raining most of the day, so it didn't seem like a good day for th beach.
(2) Schwerin/Wismar: We saw these as a day trip from Lubeck. We bought the regional day pass ahead of time on DB.com, and it was a great deal for 2 people and allowed us the flexibility to decide to go to Wismar at the last minute. In Schwerin, the castle is pretty much the only thing to do. It was a nice walk from the train station (you can walk by a pretty pond and cute shopping/restaurant area). There is a small amount to see inside the castle--not tons of stuff--and then the gardens and grounds. We had cake at the Orangery, and I would recommend that for a break. It's pretty, and the cake was good. We had lunch at Restaurant Lukas and enjoyed our meal. Because there wasn't that much to do in Schwerin, we decided to take the train back through Wismar for a quick stop. We didn't have a great plan or much of a map (or much time), so we kind of wandered around and saw the main square, which had a few cool buildings; the ruins of a church destroyed in WWII; and the harbor. I wouldn't have minded spending a little more time there (with better info), but I think you can easily see the best of both towns in one day.