Other than Koln, looking for suggestions for charming towns along the Rhein with lots of sites to visit. What is there to do there?

You'll probably get more help with this if you re-post on the Europe forum. At least I assume you're talking about Germany?

Since I'm planning a trip myself -

We are staying in St. Goar. We plan to take a cruise to Bacharach (also a good town) and ride bicycles back to St. Goar. Rheinfels is there and it's supposed to be excellent ruins.

OOPS...made a mistake! Thanks CAPH52

What is your plan? Are you driving? Are you cruising? How far up the Rhein do you plan to travel? In Mainz is a copy of the Gutenberg Bible and the Gutenberg Press. If you are traveling south and you have a car, you might want to consider a side trip to Heidelberg, which is along the Neckar River, but isn't too far from the Rhein. Strasbourg is a picturesque town, and the Black Forest is beautiful. (Again, the Black Forest would be an excursion you would need a car for).