Things to see around Wiesbaden

I would plan on doing some sightseeing in Mainz and Frankfurt while you are in the area. Both are very old, very historic cities. In Mainz, visit the Dom which is a magnificent, Romanesque cathedral, St. Stephens and its windows created by Chagal, the Gutenburg Museum, Museum of Ancient Shipbuilding, the new Synagogue, and the Isis Temple ruins. In Frankfurt, visit the Kaiserdom, Alte Nikolai, the Klein Markt Halle, the Jewish Holocaust Remembrance Wall, Teutonic Order of Knights church, neighborhood of Höchst (great farmers market and market hall on Tues. mornings until 13:00)

Other nice towns to visit that are near Wiesbaden would be Limburg, Marburg, Idstein, or Eltville.

On your way to Munich, stop in Büdingen, a medieval walled town that was never bombed in WW2, nor burned down in the 30 years war, so all of the buildings are still original. Neighboring Gelnhausen is well worth a stop too and if you have the car, drive up to Ronnerburg Castle, or stop by Hessen Park Open Air Museum. If you like Celtic stuff, drive over to Glauberg.

You can drive along the Rhine from Nürburgring to Wiesbaden. With a car, you can see quite a bit in one day. Cruising the river by boat the next day might be quite redundant.

Rothenburg odT is a regional attraction, but not unique enough to drive halfway across the country to see. You're headed in that direction on the way to Munich anyway, so if you want to see it, use that day.

As Jo mentions, the town of Büdingen near Frankfurt is also walled and almost perfectly preserved, but sees a fraction of the tourists that over-run Rothenburg on a daily basis. Although I would add that it's not really on the way to Munich. If you're looking for other well-preserved towns in the area, Bad Neuenahr-Ahrweiler, Wetzlar, Braunfels, Limburg, Butzbach, Dillenburg and Marburg are also excellent examples, although only Ahrweiler has an intact wall. Marburg in particular is a gem.

Agree w/ the idea of seeing Mainz. LOTS TO SEE there and nice pedestrianized zone made for wandering around:

Mainz, Market Square
Old town scene
Old town square
Augustinerstrasse
Mainz' Dom

Cruising to St. Goar gets you to the ruined Rheinfels Castle. The boat also stops at St. Goarshausen, directly across the river from St. Goar, where you can visit the Historic City Tower, where the wine museum is housed - and do some tasting - and where you can catch a train north to Braubach (22 minutes,) a lovely old town with some fine half-timbered buildings and where Marksburg Castle is located - the only never-destroyed castle in this zone, and one that offers tours year-round.

Thanks for all of the ideas! I confirmed with my brother-in-law that he'll be able to spend the afternoon of Tues Oct 6 and all of Weds Oct 7 with us. So I'm thinking we can drive straight from Nurburg to Wiesbaden on Tuesday morning and drop off the car (saving a day of rental fees) and then have him show us around Wiesbaden and maybe Mainz that day. Then on Wednesday we can spend the day going up the Rhine because he's never done it. I checked KD Line's site and there are still 4 boats per day in October, so we can ride it from Rudesheim to St Goar and maybe get off and check out Bacharach along the way. If we have enough time, train up to Marksburg castle, then take the train back down to Rudesheim. That means we'll have all of Thursday free to ourselves, so we'll have to think of what to do that day.

To address some of the things mentioned here: I am well aware of the costs and risks involved to drive on the Nurburgring, but it's a bucket list item for me so it's worth it. I'm an amateur racer and have dozens of hours of track time under my belt.

I realize that driving from Wiesbaden to Munich and stopping along the way will be a long day, but we quite enjoy road trips so we're willing to start early and arrive somewhat late. I also realize that some of the suggestions to visit along the way are somewhat out of the way, but I'm fine with detours.