Bayern ticket. Travel after 9 am week days, or anytime weekends for 27 euro for the 2 of you. That will get you from Munich to Rothenburg. You can't use any high speed trains with this ticket, but the distances are short enough it is no big deal. It is good all over the state of Bavaria. You can travel from there to Koblenz either with an advance purchase (nonrefundable) 49 euro for 2 or spur of the moment with a QDL ticket(again after 9 am, no high speed trains) for 52 euro weekdays, or with a Schoenes Weekend ticket 44 euro Saturday or Sunday anytime. In Koblenz, get a VRM minigroup ticket which gives 3 days unlimited travel for 44 euro or there about. It covers trains south to St Goar, north to Remagen, and along the Mosel to Bullay, plus buses.
Heading back to Munich there are 81 euro for 2 on direct Euro Cities train. That one is advanced purchase nonrefundable. Using the QDL or Weekend is 3 hours longer journey due to more changes and slower trains.
All at www.bahn.com. look under "Offers" and "Travel in Germany". The Bayern ticket is a Laender ticket.