Frankfurt-Heidelberg-Mainz Slower Senior Travel

Is it possible to take a river boat from Heidelberg instead of a train
to Mainz?

No, and this is not a very scenic area anyway.

This round-trip cruise from Heidelberg up the Neckar would be much more scenic:
https://weisseflottehd.de/fahrt/vier-burgen-rundfahrt/

On the Rhine: The Rhine Gorge begins well to the north of Mainz, in the town of Bingen. You can cruise north from there to St. Goar. This is the Rhine's most scenic cruise segment.

Here is the Scene just north of Bingen.

There are not a lot of festivals happening in March, but the Farmers markets in Frankfurt happen all year round. The ones in Höchst might be of interest as you an also see the one of the oldest churches in Germany (the Justinus Church) as well as lots of half-timbered buildings and a small schloss surrounded by a dry moat. Markets are on Tues. Fri. and Sat. from 7-13:00 and include a market hall.
For Frankfurt, the markets on the Konstablerwache are on Thur. and Sat. and are local products only, the market on the Schillerstr. is on Friday and is the largest one. Bornheim has a lovely market too on Wed. and Sat. on the Bergerstrasse and you could include a visit to Bethmann Park.
Indoors, a visit to the Kleinmarkthalle is a must.
For museums, my recommendations are the Judengasse Museum, Jewish Museum, Historic Museum, the Städel, and Karmeliter Cloister. I sort of like the Archeology museum, but it is about the only one that hasn't done a lot of translations to their texts.

There's no US Military left in Heidelberg. Most of what was there went to Wiesbaden. You can't even tell where the hospital used to be.

You may want to consider basing out of Neustadt (on the Winestrasse) instead of Heidelberg. It's more central, better than Mannheim (which is very industrial), and will give you some better options for day excursions. 4+ days in Heidelberg is probably too many, but you can get there easily from Neustadt.

Consider Speyer, where 6 Holy Roman Emperors are buried and where they have an interesting museum of Celtic history. Also, the fantastic technical museum.

Kaiserslautern and Ramstein have military facilities, but you'd need to get a pass to enter. Being retired (or current) military you'd have access.

Trier is about an hour by train from Neustadt and it's chock full of history dating back 2000 years.

When you're in Mainz make sure you visit the Temple of Isis.

Thank you all for the helpful insight and wonderful suggestions. I'll to add and edit the itinerary to incorporate your feedback.
I'm very excited to finally return to Germany after 60+ years.

Thank you again for the assistance.
Ida

Frankfurt has things to see and do, but in my opinion it is not a top tier city to tour.

Considering Frankfurt is one of the oldest cities in Germany and has played such an important part of German history, not sure why anyone would think it was not top tier. Rick has left most of the top sites out of his book because he wants Frankfurt to be the modern Germany and anything medieval or ancient doesn't fit in his vision.
For anyone who last visited 35-40 years ago, perhaps they need to come and visit again?

I like Frankfurt. I spent a day there last weekend, and I have a lot of the city left to explore. I like Mainz a bit more, but part of that is it's easier to get to and around. I've parked in Mainz and took the train to FRA because that works good for me.

The Oxfam bookstore in Frankfurt is one of the better, and cheaper, used bookstores I've found, although Heidelberg has a couple smaller bookstores that always take my money.

I miss two main tips in this conversation.

Between Frankfurt and Heidelberg you will find Darmstadt which renowned for technical university, the EXPERIMINTA ScienceCenter, Waldspirale (Hundertwasser house) and Hessisches Landesmuseum Darmstadt.

Darmstadt and Heidelberg are connected by famous Bergstrasse with Starkenburg in the middle.

btw: there is a direct bus connection called AirLiner between Darmstadt and Frankfurt airport (link to brochure).