Originally Posted by
milepig
Welcome to FT!!
From Cologne you could look at taking a Rhine cruise (lovely scenery with CASTLES) to either Wiesbaden of Mainz, and then catch an onward train from there. The only wrinkle is that although the KB line sells a day ticket which allows stops between Cologne and Mainz I can't make the schedule work without stopping overnight along the way - not a bad option except it eats up another of your nights.
It isn't you. It isn't possible to take a cruise from Cologne to Mainz in a day as the current slows you down to much going upstream. You can only do it downstream (Mainz to Cologne). As the best of the cruise is from Koblenz to Mainz, you could take a train to Koblenz and then switch to the cruise.
Originally Posted by
milepig
If you want to go right to Munchen, you can get direct train from Cologne that takes only 4.5 hours, making Cologne-Munchen very doable as a day trip. Or, you can take a side trip from Frankfurt and do a night in Heidelberg which is a very attractive city, and only an hour from Frankfurt by train.
Another option if you want to work in a little of France would be to rent a car in Frankfurt - I'd suggest the airport for easy access - and take a side trip to Alsace for a couple days. Amazing country and the food is to die for.
I agree with others who feel that Frankfurt, otherwise known as Bankfurt is not worth stopping in, so your idea of either using it as a transfer point to go to Heidelberg or using the airport to rent a car to go to Alsace is a good one. That said, Alsace is worth a week on its own!