We will leave Paris on a Saturday morning in late June. My husband wants to to to Mannheim where he served 40 years ago. We must fly out of Frankfurt Monday night. So, how do I spend a few hours in Mannheim AND see the picturesque Rhine that Rick raves about? Current plan is both nights in Heidelberg - becuase I couldn't figure out anything. Any better ideas? Could do car, boat, bus, plane, train, etc.
You need to look at a good map. Mannheim is on the Rhein, but not the picturesque part the Rick "raves" about. If you insist on staying in Heidelberg and taking a day trip to the Rhein gorge or maybe seeing it on Monday before you fly out, you will be rushed. If you want to spend the money for express train tickets, there is an IC/RB connection leaving Mannheim before 9 AM, changing in Mainz and getting to Bacharach before 11. Bacharach is a cute town to walk around in. St. Goar, 11 minutes farther away than Bacharach, has Rheinfels castle ruins. If you don't spend much time in either Bacharach or St. Goar, you would have time to go to Bacharach, then take the K-D boar to St. Goar, and come back to FRA via Mainz for your flight.
Look up schedules on the German Rail query page.
Best plan would be to forego one night in Heidelberg and spend it in St. Goar.