We'll be arriving in St. Goar a day early prior to a Rick Steves' tour. Any suggestions on how to wpend that day well?
We love Rheinfels ruins and the one hour cruise to Bacharach. The two together make most of a day after factoring in a train back to where you started and some lunch.
We loved Marksburg Castle, located above Braubach along the Rhine River. The guided tour is wonderful and they have a museum-type shop where you can purchase antique Bible pages, books, and other interesting things. If you have kids, it's a definite must-see.
Check with RS itinerary to make sure you don't visit a site that you're going to see on the tour in the following days.
I'd compare your trip itinerary and the attractions Rick lists in his Germany book. Pick the bext thing nearby that you're not going to see on the tour.
It looks like you are doing the best of Germany, Austria, Switzerland 14 day tour (it starts in St. Goar). I notice that the itinerary does not include the Marksburg or Deutsches Eck, which are close enough to see. Take the ferry across the Rhein to St. Goarshausen, and the train to Braubach (Marksburg). From there go by bus or train to Koblenz and see the Deutsches Eck, then return by train to St. Goar. If you have time, stop in Boppard and enjoy the Rhein from the river walk there.