How about going by car? You see a lot more and always can find a much less expensive small hotel or "Pension" outside the big cities. In my experience the cost of gas is less than train tickets for two. In the past i had very good experiences with the folks at "Nonstop Travel" (
Nonstop Travel). I have no connection with them other than past experiences as a customer. They are German and English speaking people to help you and they have excellent connections in Germany. They also can help you with rail passes and rental cars there. They do have an office also in Frankfurt. And I found them to have the best prices for flights, car rentals, and travel insurance.