A couple Days in the Rhine Valley

Ok... Can it be this simple? Get up in Bacharach and drive to Bingen, Park the car and boat northward to St. Goar and get off and tour the town and Rheinfels Castle. Get back on the boat and travel northward to either, Boppard, Braubach, or Koblenz (Whatever we decide is the final destination) and train back to the car in Bingen... If this is a good plan? Can you park at Bingen and leave the car for the day? And can you get off at St. Goar (or any town for that matter) and get back on the boat with the same ticket or do you buy one for each leg?
Thanks.

Driving to Bingen and getting on the boat there is a good idea, but I would stay on the boat the whole way to Boppard and then head back by train, getting off at the towns you want to visit. There are more trains than boats, making it easier and more efficient time wise. You won't need all day to tour Bacharach, so spend some time visiting some of the castles you want to visit. Take the ferry across the river to visit the Marksburg Castle, or you can also visit Rheinstein. You have plenty of time. Even visiting St. Goar and Rhinefels won't take all day, though a few hours are good to set aside to explore this massive fortress and also the churches that are in town.

"Get up in Bacharach and drive to Bingen, Park the car and boat northward to St. Goar and get off and tour the town and Rheinfels Castle." So far, mostly OK. See note about car below. "Get back on the boat and travel northward to either, Boppard, Braubach, or Koblenz (Whatever we decide is the final destination)" I wouldn't bother with Koblenz. Boppard is a nice place if you prefer to just hang out, maybe visit a winery, stroll the promenade, check out the half-timbered buildings, have a meal at one of the outdoor cafes on the Marktplatz, take the chairlift ride up and have a beer overlooking the valley, that sort of thing. I would venture up to Braubach only if you plan to tour Marksburg Castle. Because your time is limited, I would end the cruise altogether in St. Goar and proceed to Boppard or Braubach by train (for Braubach, cross over on the ferry from St. Goar - it runs all day - and take the train north - that train takes only 20 minutes to Braubach.) I would also suggest that you not drive to Bingen. The train from Bacharach to Bingen Hbf takes 12 minutes. But that 12-minute train ride earns you a 20% discount on the cruise (normally 37.40€ for two, so about 30€ instead if you show the train ticket at the KD dock.) A Rheinland-Pfalz ticket (daypass for the trains, 26€ for two, buy at ticket machine in Bacharach) covers your afternoon ride to Boppard, or Braubach, then back south to Bacharach, and it will also cover your morning train to Bingen. The R-P daypass is good anytime on Sat or Sun, after 9 on weekdays. (It also covers your ferry crossing in St. Goar if you go to Braubach.) You should make time for a stop in Oberwesel (5 min. by train from Bacharach) if possible. Among other attractions, the old town wall is walkable and offers great views.

Thanks for these suggestions. I appreciate your help.

Hello Cynthia. I recommend that you do not drive a car to any of those places. Day One : Leave from Koln at the earliest possible time. Ride in trains, via Koblenz, to Moselkern. Walk from there to the castle Berg Eltz, and return to Moselkern. Your baggage can be put in lockers in the Koblenz train station. Ride in trains from Moselkern to Koblenz (retrieve baggage) and to Bacharach.
Day Two : Ride on a K-D ship on the Rhine river from Bacharach to St. Goar. Go to Braubach for walking through the castle Marksburg. Return to Bacharach. When you walk through the castle Marksburg, you will be in a group of probably 20 people with a guide. It is very likely that your guide will talk in the German language. If you do not know the German language, that is not a problem. Before you enter the castle, ask for maps of each floor in the castle, with words in the English language printed on it. On Saturday mornings, very many German people ride in trains to Koblenz.