Updated Germany trip plan:
Dec 18: Arrive in Munich at 8AM, spend day and night in Munich
Dec 19: Rent car and head to Hohenschwangau to tour Hohenschwangau and Neuschwanstein castles, drive to Heidelburg.
Dec 19-20 Visit Thingstatte, Heidelburg Castle, Church of the holy Ghost. Leave by 5PM on 20 Dec so as to arrive in Rothenburg in time to check into hotel, and go on the Night Watchmen Tour at 8PM
Dec 21: Spend day in Rothenburg seeing the sights and shopping
Dec 22: Leave Rothenburg, possibly head up to Wurzburg to see Marienburg Fortress (Festung Marienberg), St. Kilian Cathedral (Dom St. Kilian), and eat and try wine at Burgerspital (haven't decided if it is worth the 45 min side trip yet). Head to Salzburg Austria
Dec 23: Spend day in Salzburg, Austria
Dec 24: Return to Munich, drop off car and head to Rafz, Switzerland to spend time with family.
We do know that if we plan to head to Austria that we must acquire a permit sticker for our rental car or else face big fines. The places that are listed I have gone online and the ones that we want to get inside of to tour will be open during this time of year.
Just some small comments:
I would be careful with driving times. The traffic jams that happen just before Christmas are legendary, looking at the calender I think a good candidate for some serious traffic would be Sunday afternoon, the 20th Dec as well as the days just before Christmas after 4 pm when people get off from work. I would thus try to put any driving into the late morning.
I would also spend two days in Salzburg and more time in Munich if possible. I'm sorry, but I can't see the attraction of Heidelberg - you will be going to Rothenburg for the wood timbered houses and you will be seeing a great castle in Salzburg - why subject yourself to more of the same in Heidelberg? I would probably skip Heidelberg and go straight to Rothenburg.
Dec 24: Return to Munich, drop off car and head to Rafz, Switzerland
That is something I would rethink. If I was you I would drop the car of earlier and take the train from Munich to Salzburg and then the train from Salzburg to Rafz. If you only want to see the city a car is completely useless in Salzburg. Saves you the hassle of returning the car on the 24th and trying to catch a train in the afternoon of the 24th. I think it will be better if you hop on the train to Rafz in the early morning. Schedules during Christmas eve can be a bit erratic - train drivers and conductors want to be home on Christmas eve too.
Just a bit on the car sticker you mention. You can buy 1 at just about any petrol station or post office.
t_maia has covered the rest.
Thanks for the advice. I think the big draw to Heidelburg was seeing Thingstatte, but since we are also going to Italy and Rome later on in the trip I have a feeling that we might be able to see an old outdoor theater there. As far as the high traffic times and returning the car before heading to Salzburg I will talk to my wife about that and try to update our plans when I get a chance.
Once again, thanks for the advice