We will be a River cruise on the Rhine and Mosel river Aug 2015. What are must see places?
If you have the freedom to stop where ever you'd like, then enjoy that and stay as loose as possible. There's beauty all along this route and you may find your own treasures as you go.
Much of the beauty of the Mosel River is in the small towns and vineyard covered hills. Bernkastel-Kues and Cochem are both popular, a bit touristy, but also have old castle ruins to explore and lots of wine tasting which is a large part of this region. If you have time and the ability to be spontaneous, you can explore some smaller towns in between. (I've listed several smaller towns in my Mosel wine region post.)
I don't know the Rhine as well. Koblenz is a must stop though and if you have time, do explore Cologne (Köln). It's a remarkable city with a lot to see. You'll see the Kölner Dom from the river. It's free to explore. From here, it's the perfect spot to cross the bridge and walk along the waterfront on the opposing side, then cross over near the Schokoladenmuseum and make your way back weaving through the old quarter and towards the newer shopping areas just east of the Dom.
Ok... I'm adding this little bit here after talking to some friends living in Germany, so here is their 2-cents worth.
Amy says that Romantik-Schloß Burg Rheinstein is just gorgeous and after seeing pictures, I'd agree.
Raquel says "Koblenz to Bingen am Rhein in the Middle Rhine has the nicest scenery and concentration of fortresses. That stretch is actually a Unesco world heritage site I believe. One of the castles is actually a hostel you can stay in. See http://en.m.wikipedia.org/wiki/Middle_Rhine"
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