Rhine River on a Sunday

Those are all towns filled with tourists, so their souvenir stores are going to be open. St. Goashausen only has a few restaurants and hotels, not a hot spot for tourists. Rüdesheim is always busy.

If you have an extra day, consider going to Eltville and Eberbach Monastery since you have seen all the castles already. Am assuming you mean Burg Rheinfels and the Marksburg?

Thanks. Have not been to either so maybe that’ll be the plan.

Have already done most the castles and museums.

So how about something different?

Have you been to Boppard (only a few minutes north of St. Goar) and taken the chairlift ride to the lookouts up there?
It's a short walk of several minutes from the top to Gedeonseck terrace:

There is a nice walk in the highlands above St. Goar that takes you to Oberwesel (5 min. from St. Goar) in about 2.5 hours. The scenery is great.

Look for the little detour to the Günderodehaus - nice spot for refreshments - see photo with Oberwesel in the background. Oberwesel itself can be the base for a (steep) walk up to the G'haus. Or you can take the walk around O'wesel's town wall.

Thanks, have not been to Boppard either so will give that some thought too!

https://www.marksburg.de/en/

If you have not been to the Marksburg i highly recommend it. It takes 20 minutes by train from St Goarhausen on the east bank of the Rhine. It is open every day.

https://www.bahn.de/en/view/index.shtml

Agreed, well worth it. Thanks though.