Going from Paris to Hamburg

For downtime, I highly recommend a few days on the Mosel river somewhere. It's quieter than the Rhine, and there's a flat bike path along the river for as far as you want to go. Vineyards, good food, pretty little towns, and of course Burg Eltz.

We hugely enjoyed breaking our trip from Paris to Osnabrück with an overnight in Zell an der Mosel, in this lovely baby castle hotel: Hotel Schloss Zell. It doesn't have a train station right in town, but the nearest station was a 5-minute cab ride away. Zell is a lovely town on a big bend in the river. We climbed up the hill behind our hotel and wandered through vineyards with gorgeous views and the odd old stone guard tower to climb for even better views. Several nights would have been even better.

Cologne doesn't get a lot of love on this forum, but I love it a lot, as I lived just outside the city for a year as an exchange student. There's lots to do. The Roman museum next to the Cathedral is fantastic. It's built around an original in-situ Roman mosaic floor - they just sited the museum to fit around it! Arriving by train over the beautiful triple-arched railroad bridge and seeing the Cathedral makes me choke up every time. Also worth a couple of nights, IMO.

And you should fit in some days along the Rhine too. Are you a music fan? The Rheingau Music Festival starts June 24 and runs all summer in various historical venues - abbeys, castles - with all sorts of concerts, mostly classical and jazz but other genres too.

So many great choices...I can offer more if you'd like. Hamburg is wonderful to visit too. Have a fabulous trip

Cologne was nice, but not quaint. Cathedral is worth seeing. Just know that other than the cathedral, it is all new as it was pretty much destroyed in WWII. I think a day would be plenty there. I would also recommend the Mosel and then Koblenz is at the confluence of the Mosel and Rhine. I haven't been to Mainz, I believe its pretty big. You might consider some of the wine villages along the Rhine like Bacharach, Assmannshausen. On the Mosel, I'd recommend Winningen for a little wine village you could visit from Koblenz. From Koblenz there are a couple of castles you could tour that are close. Marksburg and Ehrenbreitstein.

We really enjoyed:
Bremen - The altstadt is really cool. We were there for a family reunion, so we were on a whirlwind tour of the city. I wish we could have explore the Schnoor a bit more. But that will be for next time.
Bremenhaven We enjoyed the immigration/emigration museum
Lüneburg - This city was soooooo cute!! We enjoyed wandering through the town and going up in the Watertower.
Cuxhaven - which is not in between, but still a fave. You can watch the ships leave the Elbe river into the ocean. There is a viewing grandstand that announces the ships. It helps if you speak German, but cool either way. Also there are beaches there you can dip your toes in the ocean.
Stade this little town is between Hamburg and Cuxhaven
Lübeck (which is not in between, but still a fave) We had fun here shopping. We enjoyed the Willy Brandt Museum and the various churches. Wish we had more time here.

Are you going by car or train? Some of these places might be a bit out of the way for the train (Stade and Cuxhaven).

Thanks to all for the amazing suggestions! This is why I love this forum!! It's looking like 3 nights in Zell is going to be my choice, leaving me plenty of time to wander the vineyards, see a castle or two, try for the music festival, and get in plenty of relaxation time before heading to Oldenburg to visit friends. I'll have to plan another trip to see the other marvelous suggestions!

Delighted to offer suggestions and happy you've found something you like. We loved Zell, it's quieter than some other towns but really nice. I'd love to hear your thoughts afterwards via a trip report.