Traveling to Frankfurt

Take a regular train (RE or S-Bahn) from Frankfurt Flughafen Regionalbahnhof to Frankfurt Hbf. They are more frequent and cheaper than the ICE (InterCityExpress), which depart from Frankfurt Flughafen Fernbahnhof. ICE is a high-speed long-distance train, so you're not the targeted audience for this service.

Thank you!

yira949, just to note that Frankfurt is not on the Rhine River: you have to travel some distance to a town from which you can catch a cruise. That might impact your choices.

Do you mean Hotel Isha? If so, you will need to take the S-8 or S-9 from the airport to the Hauptbahnhof, and then the S-3, 4, 5, 6 from the main station to Frankfurt West. A better alternative is stay on the S-bahn coming from the airport and get off at the Hauptwache and then take U-6 or U-7 towards Hausen and get off at Kirchplatz.

Starting 24 Oct. the S-8 and S-9 will be leaving and arriving in the long distance station instead of the regional train station as they are working on the tracks. The Regional station will be closed from 24 Oct. to 14 Dec.

Take the once an hour train (07:53, 08:53, 09:53) from track 24 or 23, to Rüdesheim, walk to the KD dock, show them your train ticket and buy a ticket to go to St. Goar. Take the train back.

Thank you so much for your reply. Do you advice renting a car vs train? we were thinking of going to Heidelberg maybe the next day.

Reply to Stan...
Thank you. What city do you think will be better for a short Rhine River Cruise?

Yira, I just told you where to go to take a Rhine cruise.

Unless you have special needs or mobility issues, there is no need to rent a car for 2. It's expensive and a headache. All of the things you want to do are easily done with train or bus. Heidelberg is accessible with either bus or train from Frankfurt.